Got a saw and some drill bits?
I hadn't seen one for sale commercially, but with some time and effort, you can make your own. A Jigsaw and/or band saw will help.
It would be better to make one out of hardwood than plywood. Better for an evenly dense wood like Cherry or Walnut than Oak (sense Oak has porous rings, for example). Any wood will swell when wet, which will prevent leaks.
Take a gasket and draw the outline onto the wood. Drill the holes first - sense the four holes are maybe the biggest issue to get correct. If you drill a mounting hole in wrong location, you will need to start over. Once the four holes are good, then drill a hole in the middle and use a jigsaw to cut out the middle part with, A course file to smooth things up, then sandpaper to finish off with.
Don't use any finish on the inside - for gasoline or cleaners will remove any finishes.
I would put something on the outside of the wood, and several layers of engine paint might be best. Wood stain might also work. A urethane finish will not work (it will desolve in no time)! If you got any paint on the gasket surface, then sand it off (and don't leave paint on a sealing surface). Use a thin gasket on top and bottom and you are set.
It will not matter if the wood plate is warped or not - for it will straighten out once tightened down. The important part is top and bottom are parallel to each other.
If you are using a divorced choke (choke attached to intake manifold) - then a person will need to find a way to get the choke adjustment down correct. Even having a slightly thicker gasket is enough to get the choke settings off - not counting a 1/2 inch (or whatever height) spacer added.
One last thing. you might need longer fasteners (bolts or studs).
I have a 3D printer and could make one - but I don't think plastic would be adviseable here (melt from heat or melt from gasoline contact).
BudW