Haven’t done it but seen a lot of others work. I have build a lot of custom marine seats tho. From what I’ve seen use a wooden base on top of seat frame. Then cut out where the customer/passenger will sit and staple I’m webbing cross weaved together. I’d probably use a thinner layer of foam with higher density to cover the initial board with the webbing, just so it doesn’t cut/tear when webbing flexes. Then use layers of foam on top the suits the job and customer for comfort. Don’t have any pics but I’m sure someone else will chime in with pics and more experience. Just a start/idea good luck and hope it all works out.

I did a bench seat for my personal C10 last fall and tried this method out. I used the stock seat frame and put 3/4" plywood for the base and back rest. I used 2.2 density 50lb compression foam on mine and shaped it myself. It was a little on the stiff side when I first did it. But after you put some miles on it the foam seems to break in some and get softer. I'm pretty happy with it, but I wanted the firmer feel and never liked how old truck seats can be a little bouncy. I'm not sure what the density and compression is on the Snowden foam so that might make it feel a little different. I've see quite a few guys do it the way Justin mentions also with cutting out the wood under where you sit and weaving jute webbing in that area. I was going to try this method out on my next one.

Thanks guys, your suggestions are basically what I pictured. I’ve done similar things for boats, restaurants and early hot rods but never on a springed style seat.
Thanks again!

Here's one of several options to create a flexible seat base without the old preformed or zigzag springs:
http://atrim.com/Page185.html