Recommend a multi-driver race seat

Car is an E36 M3 which most likely will only ever see HPDE.  Maybe some time trialing.

4pt welded rollbar and harnesses are on the way.  I'm shopping seats.

My dad is my co-driver; I'm 5-10 175, 33" waist.  He is 6-0 27X? 42" waist.

I want two fixed-back seats in order to take passengers where allowed, and instructors when required.  Dad is the greatest common denominator for both sides.

So far he has only sat in an OMP HTE-R XL which we had access to.  It measured 16-1/4 across at the lap belt holes and fit him snugly.  Many, but not all, seats for sale online list the inside width across the hips, with M often 15, L 16, and XL 17.  I fit well in most 15" seats.

Based on the OMP I think we need a 16.5 or 17" width seat.  I'm not seeing many reasonably-priced options for that, outside of aluminum setups like Kirkey or Ultrashield.

Racequip offers the FIA composite 968 which is 17" wide and $400.  Seems about right, I contacted them but they don't have any currently in stock at any dealers for us to try out.

So I have two questions:

-Can you recommend a wide seat for us to check out?

-What have been your experiences with padding/velcro/molded foam inserts to fit smaller drivers into larger seats?

I really like my RaceTech 4119 (the "WHR", so wide but normal height), it seems to fit a lot of people.  It's quite a bit more than $400 though, even in the fiberglass version.

I bought a Cobra Monaco Pro.

With a fairly "sturdy"  (5-9 225  I wear 36 jeans I wear an XL or XXL shirt depending on brand) build myself, it was about the only seat I could find that fit in  Miata and fit me.

According to this schematic it shows at just shy of 16 inches, but it feels quite roomy to me. The Cobra is also quite roomy in the shoulders, which is important for me.  I'll try to remember to put an actual tape measure on mine tomorrow and report back.

EDIT:

To provide a bit more info. My car came with a Sparco Sprint, which was way too small. I bought a Momo Start which appeared, according to provided dimensions, that it would fit me, but turned out to be nearly as small as the Sparco. I also thought I would be looking at Kirkey seats, but stumbled upon a guy at the track with the Cobra, and it felt nearly perfect.

I'm about your dad's size and I have a 17" Kirkey intermediate road race in my Miata. With the Kirkey padding it is comfortable for the typical 20-min HPDE session. I trailer it, so not sure about longer sessions. 

I don't have direct experience, but GRM did an expanding foam seat for one of their cars. https://grassrootsmotorsports.com/articles/how-form-your-own-custom-foam-seat-insert/

My friends who endurance race use foam inserts or back pads to make sure everyone is snug. 

I have Corbeau FX-1s (can't remember if they are Pros or not) in the wide width.  I'm 5'-10" at about 265 with a 44" waist and I am comfortable in it.

Buy a seat that fits you and then as mentioned buy a seat insert kit. There are a few brands out there.

Those of us with formula cars have to pour a seat.

The budget option is making one form expandable A/B foam from a place like Aircraft Spruce.