I used a second seat out of a mid 90's dodge mini van & built a frame to mount it. When i got my truck, the factory seat riser/gas tank was M.I.A. Figure your seat height carefully.. i'm still adjusting mine after 6 mo.
i used the 3rd seat from a caravan--had 3 seat belts --top flipped back- was adjustable front to back---i reworked the bottom mounts on it to bolt it down--i also found if i raised it a lil it gave me more leg room---seat is comfortable --right width-and back is narrow enough to add to leg room---gives me more leg room than a stock one
I would make one from 3/4" plywood, make a template first out of cardboard. Use med firm foam for the seat area & med soft foam for the backrest. In the seat area I would cut out a 10"x18" piece on each side, out of the 3/4" plywood & use webbing across to give it a bit of a flex. This way the seat will fit the cab the way you want, without a large gap on the sides. Usally 3-4" thick foam for the seat & 3" thick for the backrest. You can cut & shape the foam with a electric carving knife. Once you have the fit you like, cover it yourself or take it to your local auto upholstery shop.
I used a mid '80s S10 seat. I narrowed the front of the seat frame 3 inches on each side. By narrowing the seat, it fit the rear to front taper of the '40 cab. I removed the original seat riser and mounted the seat on a pair of seat tracks out of a '88-'99 Chevy truck. It worked out really well because I retained the full travel of the seat track. That enables me to slide the seat forward for behind the seat access. Of course I had to have the seat recovered after the modifications.
I used a back seat out of an '89 Tempo in Henrietta the '38 Ford pickup (stock '40 seat is same as '38). The back cushion was too tall, so I peeled the upholstery back from the foam, cut 4" off the bottom of the back cushion, pulled the upholstery back and refastened with new hog rings, looks like it was born that way and fits the cab nicely.
Dodge Mini Van, this is in a 35 ford P.U. same seat as used in a 40, it has the gas tank/seat riser in it, it ie mounted to the top of the tank (the tank is not used)
This is a cut down S10 seat I had in my '40. I also used a Chrysler mini-van seat in my '46 Ford pu (same cab as '40)
Ranger pickup bench seat works great. Back at 48" fits as is, seat bottom needs to be norrowed 1 1/2" on each end.