Clark Rubber in AUS sells Closed Cell foam of various grades and thicknesses. It is relatively expensive. They also however sell "Stadium Seats". for $8. I can tell you that one if these (Lt Blue / 20mm thick x 360 x 450 mm) will make 1 x full seat pad (carved and shaped for long marathons eg HCC) and 1 pair of hip pads which fit a Swordy. Even a bit left over. I fastened my ski pads in with decent 50mm wide Double Sided Tape and then Gaffa Tape over the top. Have been there for months now. Easy, light, neat. I was careful cutting them neatly with a new blade and placed them lightly until I was happy with the location. I still only just touch them (31" waist / 68kg) but they do give reassurance and better feel in cross chop on the Harbour etc. See pic of new Stadium Seat. Can carve, sand, shape, tape, glue etc. Dense yet flexible and doesn't retain water.
Current: Carbonology Vault, Carbonology Boost LV, Vajda Spirit K1
Previously: Fenn Swordfish, Sladecraft Sonic, Older K1, Elliott Vindicator, Elliott Marauder,