I tried for a while to make a seat out of alloy sheet, and failed. the shape is just to complicated! in the end I decided on an alloy and glass fibre approach. I lifted the tank slightly so the finished seat will have some clearance, and then covered it with duct tape, followed by some tin foil to act a a release. This actually worked quite well! Cutting the foam to shape took a while and is a dusty job, but with the fan blowing on me and out the door it wasn't too bad. The key to using resin is the mix ratio... I used 2.5% as it was a cold day, in the summer that would have gone off in 5 mins! I will send it to the trimmer to do his magic... this is one of the few jobs I send out.
