Made the seat pan this weekend. I'm partial to fiberglass. Not sure if anyone's ever documented on how to make a fiberglass pan so here's how I do it.
Mask off everything you dont want ruined :-D
Go to the hardware store and get some aluminum ducting tape and tape over the masking tape.
At this point, if you haven't already, put on some nitril gloves. You do NOT want this stuff on your skin. It's REAL sticky and will stay with you a long time. I'm pretty sure it isn't good for you either. And who wants to walk around with some fiberglass stuck to your fingers?
Use some paste type car wax and HEAVILY wax it (but dont remove the wax...leave it on there or else you're going to have a hell of a time peeling the finished glassing off). Then cut some fiber glass fabric roughly the size you're wanting the finished pan to be however leave plenty of hang off. You're going to be trimming it to size later.
Cut some of the fiber glass matting into 2 x 4 inch strips and cut some into 2x2 inch squares. Mix up the resin. I use a foam brush to distribute the resin. I pore a good portion of the resin onto the fabric, spread it out, and then start layering the glassing in strips but make sure to overlap some. I make 3-4 layers which comes out to about 1/4" thick product. It's real firm after it drys. I let mine sit over night before final shaping.
After its dry pry it off carefully and shape it. To shape it I use a metal cutoff wheel. Sketch out the form and then cut on the lines. Then use a die grinder wheel to shape it further. For final shaping use a 100grit flap wheel.
I also setup the mounting points. I neglected to get a picture of the final product but will try and get one tonight. For mounting I'm using M6 bolts with 2 mounting points where I can reasonably get to them to remove the seat for accessing the cowel mounting bolt. To stick the bolts to the seat pan I used JB weld but you can also mix up some resin and use that.