I just got word this morning that my radiator is due to be replaced for free.. but I still want to go from a mechanical fan with clutch to fully electric. I think I have the skills to bend aluminum and cut the holes necessary to mount both the shroud and the fan.. but the question is, what gage aluminum should I use to keep flexing to a minimum, but still be easy to work with using simple hand tools and brakes?
What fan are you using? Most electric fans that I have ever seen come as an assembly with the shroud.
Don't worry too much about going too thin, it just has a little aerodynamic load on it. The thickest gauge that you're comfortable working with hand tools will be more than thick enough.
If I were making a fan shroud out of aluminum, I'd use something just thick enough that I can't bend it too easily with my fingers, for lightness.
I used 1/8", probably overkill, but I did not want it rattling like a tin can. Piece of advise, if you have a water jet cutter near by talk to them about cutting the shroud template and holes for you. Often times they have extra space on cut jobs and can squeeze your project on a sheet.
I did my shroud for under $120 excluding fans.
1/8" is definitely overkill. 0.050" should be plenty if there is any shape other than 4 flat sides. Add breaks or beads to get the stiffness and keep the weight down to something manageable.
1/8"!? Well if you want to build a lighter one later on, you can repurpose this fan shroud as a skid plate! 
Summit and Jegs have kits.
how does $13 for material sound? very easy to work with. Two halves, a top and bottom w/ bent seam edge for sealing and to make it easy to remove. Edges can be pretty sharp, so just be a little careful there. http://www.homedepot.com/p/Amerimax-Home-Products-20-in-x-10-ft-Aluminum-Drip-Edge-Flashing-68320/100071450
1/8" is not as crazy as you make it out to be.
Weight is not a huge issue because the majority of the material is cut out with the holes for the fan.
.050" sounds insanely thin to me.
I'm sure the ideal solution is somewhere between .050" and 1/8"
All the shroud is doing is providing a cavity for the fan to create a low pressure zone. That doesn't require much in terms of material strength. Plastic OEM ones often integrate the fan(s) so that they're a mount as well as a shroud, necessitating more structures. So it depends on how the fan is mounted.
Dodge intrepid fans. Shroud, dual two speed fans, oem reliability and engineering. 20 bucks at the salvage yard. I prefer the mid 90s version. Cools my 425whp 70 duster with ac in July in the south with no issues. Oughtta do wonderful on a stock land rover in Jersey.
Dusterbd13 wrote: Dodge intrepid fans. Shroud, dual two speed fans, oem reliability and engineering. 20 bucks at the salvage yard. I prefer the mid 90s version. Cools my 425whp 70 duster with ac in July in the south with no issues. Oughtta do wonderful on a stock land rover in Jersey.
Along these lines the Ford Taurus fans/shrouds are very common in the Jeep/Buggy/Offroad community.
I was planning on the Ford Taurus fan. I know they seem to be well liked in the offroad world.. which is much more extreme than anything found on the highway.
I contemplated building one of out of epoxy and glass.. but I can't find the temp that West System epoxy bursts into flame
Ill throw another junkyard option, volvo 940 and 960..nice and self contained, the relays and sensor is right there next to it so you can snag it all at once. Even has little flappers so at highway speed the fan doesnt cause excess drag.
Make sure your shroud is as far from the radiator core as possible. I don't recall the material thickness on ours, but it's definitely not 1/8" 
Give the guys at Flex-o-lite a call. I have used them on several projects and they were great (as were there fans) Probably want to get your hands on what ever radiator you are going to use and then get the dimensions for it as well as the back spacing to the motor.
mad_machine wrote: I contemplated building one of out of epoxy and glass.. but I can't find the temp that West System epoxy bursts into flame
I wouldn't worry about that, the number (for the cured product) is probably higher than the fiber filled nylon a lot of OEM fan frames and shrouds are made from, and the tanks on the radiator.
I have found that you can deform a "coupon" of epoxy made from West System if it is exposed to 230 degress for an extended period of time.
If that looks like it would work, building a shroud is very easy for me. I could even use part of a junkyard one as part of it's "core"
In reply to mad_machine:
That I would worry about.
why do you think you need a shroud for an electric fan? many oems never bothered with a shroud of any kind around their electric fans other than the outer edge of the fan blades and didn't have a problem keeping things cool.
example, 1994-1996 Caprice:
I used .040 aluminum for mine. Works great.
I was looking at another guy's E36 Ford 302 swap page. He was building a drift car. Beautiful, tidy work. Then I got to the slide where he had put a shroud on the radiator fan he had set up as a pusher.
Shrouds are always better for efficiency...for puller fans anyway. Avoid running fans in a pusher configuration wherever possible.
GameboyRMH wrote: Shrouds are always better for efficiency...for puller fans anyway. Avoid running fans in a pusher configuration wherever possible.
Shrouds are great for the efficiency of the fan, but not necessarily the total system. Once you are up to speed, a fan shroud can actually be a restriction on airflow through the radiator.
Shrouds are for making sure your car doesn't overheat in 108 degree stop n go traffic with the AC on full blast.
The air speed across the radiator is quite a bit less than you might think unless you've done a lot of work on post-radiator ducting. I've seen fans (and shroud) make a significant difference at 75 mph. Just for fun, put a strong fan in front of your car and then try to spray water into the grille from a few feet away. Most of the air goes around the car. You need those fans.
If you run without a shroud, then your fans will either pull air from around the edge of the fan (doing nobody any good whatsoever) or they'll only pull through the part of the core directly in front of the fan. That means about 50% of your core (or less) will not be seeing airflow from the fans.
My BMW has a shrouded pusher on the AC condenser, but it's got bypass flaps on it as well so they can open up if there's more pressure on the wrong side of the shroud. We do the same on our puller shrouds.
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