Yes, DH has learned that when certain things are needed this type of approach can work well. For example when tires need replacing on a scooter, you can check with your local hardware store if they have the same type of tire or can order them. On many models, they are the EXACT same tires as used on certain wagons or other such tools and buying them through a hardware store they are half the price or less. When he wanted a custom modification made that involved electrical wiring, he went to one of the places that does automotive custom lighting and such, as they work with 12 volt batteries all the time and they principal is the same, again a fraction of what a dedicated mobility repair shop would want and they were able to come up with things a dedicated shop either can't or won't do, sometimes due to restrictions of them being an official repair person and agreeing to certain rules. Building a custom seat wouldn't be far off from what these types of places do anyway, so again, this would make sense to take this approach with it.Have you considered talking to someone locally who does boat or automotive interior work? If you can find a shop that does custom work they can likely help you.
We too often box ourselves into thinking we have to select the product or service specifically geared toward mobility devices when sometimes we are better served checking elsewhere.