Keep in mind that only the front seats come from Vans with foam and all that’s needed is covering. The rear seats in a -10 are very similar to other Vans models - you build a seat back from aluminum and the rest (foam, covering) is up to you. If your rear seats aren’t satisfactory, the issue is likely the foam and sculpting done by the builder or farmed out to an upholstery shop. These can be everything very simple slabs of foam rubber (worst) to multiple layers of differing densities of confor-foam (best). Talk to a good upholstery shop, Carl mentioned DJ and she does good work. AeroSport Products does exceptional work and will make your seats feel like they came out of an upscale auto.
RV-10 #2 waiting on DAR inspection