This has been discussed over on atomic zombie forum.

www.atomiczombie.com

I used outdoor furniture material that I got from an upholsterer here in
Aus.

Here is an excerpt from a post at atomic zombie that might help if you
are in USA.

........ mesh called "Pet Screen" at Home Depot or Ace Hardware. I think
it is made from something called Phifertex. In my case, I used a similar
material that was from an old canopy my wife used to create shade at dog
agility trials. If you search the old forum for "mesh seat" you will
find 3 pages of threads talking about mesh seats in some way. Some
patience is required to search the old forum (takes a few minutes to
return the results, but worth it).

Here is the company that makes Phifertex

http://www.phifer.com/Consumer.aspx.

You might want to have a look at this.

http://forum.atomiczombie.com/showthread.php?t=1468

Have a look through and you'll see some pics of my Euromesh seat clone.
I built it for a trike but now I'm doing one for my Bentech too.

The Bentech is a really great bike. If you are building from scratch,
one thing I noticed is the chain is a bit close to the chainstay on the
smallest sprocket. If I did mine again I'd angle the chainstay up just a
tad more. Also I'd make the bend in the main tube a bit less. I suspect
the muffler shop overbent mine a bit but I've not checked. It just seems
to point down a bit far. I'd like it to be parallel to the ground.

I made my original seat 15" wide and with a lumbar support. You can see
that if you look in the photo section at John's Bike.

Keep us posted on your progress,

John Lewis