The fabric most often recommended for DIY seat replacement is Gore Phifertex. It wears well and doesn't cost much. The picture below is a seat I made recently for my Greenspeed GTO. The original Phifertex seat lasted about 30K miles before it wore enough to look a bit ratty. You might use the same system GS used, a simple rectangle held in place by 6mm shock cord. It is sold by the yard 54" wide and comes in a variety of colors. One yard was enough for two seats, one for an earlier trike and this one. You can buy it online for about $20 a yard or if there is a marine supplier nearby, they carry it too. I bought a 50 foot roll of 1/4" shock cord for $18 by special order at Home Depot. The brass grommets are there too. If I were going to replace that seat, I would make it so that the seat fits with enough of an opening to lace it with the cord instead of using zip ties since this allows you to adjust the tension on the seat, not just hold it in place. A friend sewed the most recent seat for free on an ordinary sewing machine. The earlier seat was sewn by a leather repair shop for $20 which I thought was rather expensive for such a simple job. Sew first, add the grommets later of course.
