I'm about to start making the back seat but have little experience of using a sewing machine. For those who have already done this -I have a few questions; did you staple the vinyl to the plywood base and back or create a draw string type of fit? Can you still get the battery out when the base is screwed down?

Just staple the ones we do into the plywood and if needs be a few tacks, you need scrim between the foam and the material to stop it binding and puckering with use.

Good point - I will get some scrim. The rear seat area isn't flat anymore and looks like I will have to build up the edges with 25mm batons which will also be tapered towards the rear. I like the idea of hinging the seat base to access the battery, but it could be overkill if access can be made from the front opening.

You could put 2 bolts through the ply before you trim it  to act as location pins through holes in the tub.