I finally got a real set of magnetic numbers. All that money I spent on tape....
Got these from I-Zoom, they are the "Indy" style.
Granted the first ten or so years were with Fieros and Corvettes, so magnetic's didn't work.
Edit: This thread is useless without pictures! 
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Congrats. Yeah, I need fresh ones, too.
Scene: GLDiv Rallycross National Challenge, 2015. BFNowhere, Michigan. Driving a borrowed Miata. Have no prepared numbers but do have a roll of magnetic sheet from McMaster-Carr and a knife. Completely forgot how to draw a five, so Knurled channels his inner Strong Bad.
"First, you draw an S. Then, you draw a more different S."
Dude with an Escort a few feet away overhears, says "I like it!"
Still got those numbers. I have no idea how I managed to make numbers that good. Normally I would just download from the Detroit SCCA website and use a printout as a stencil on some crap magnetic sheet from Pat Catan's, which is why I always had to tape my magnetic numbers to the doors..
I recently tested the Lowes magnetic duct covers idea.
They stayed on at 85 mph- that works for me 
I had the duct covers before this, never could get them to look good and fell off a couple times.
I used to make my own. Then I started making them for others. Nice Little profit for a while.
In reply to Gearheadotaku:
Wow- sharp car! Good choice on the font for the numbers too.
In reply to Gearheadotaku: That's funny, after my first autocross event, i bought a big magnet with my class and number on it. 
It didn't look as good as individual magnets, but was much easier and faster to put on.