Plastic was easy: heated it with heatgun and bent. I bent it slightly over and pulled back so I get almost sharp corners. That plastic can be bend with same machine you bend sheet metals. When using that machine corners will be really sharp. I used only hand tools so corners are almost sharp
Foam is frost insulator used around houses here in Finland. I don't know if it's available in US but I believe there's some similar stuff? It's very light and strong and you can shape it with knife. I'm considering doing next seat's frame completely with that insulator.
I think I'm going to use same skin used in Lynx (BRP product nowdays) seats. Those sleds are manufactured here in Rovaniemi so it's available here.
Foam is taken from Ski-Doo Scandic seat. It feels slightly too soft without skin but they say when you put skin over it tighten a little. So hopefully it will be perfect
I forgot to say: My seat weights about 6 lbs.
