Best to use Cordura, water proof and tough.
You've got the basic idea, the webbing takes the weight, foam to make it comfy, with the fabric to finish it off.
Always like to help students... with projects etc....
As long as you dont mind the seat in Grey Cordura we can supply the fabric and Webbing and edgebinding tape that you need.
Just cover the postage that will be fine.
(You've also caught me after a couple of beers so feeling in a generous state at the moment! lol)
Cheers
Jon
http://www.buggybags.co.uk