Well, I would'nt want to go touring, but.............

Foamsolutions on ebay provided a block of 45mm High Density Closed Cell Foam.

• Easy to work with. Treat it like a soft wood.
• Lightweight.
• Waterproof (not that it matters after its covered)
• Cheap - under £10 delivered, and enough for two seats)

However, I couldn't recommend it if you intend traveling for any distance. I'm sure I've sat on softer rock!

A couple of hours work, and the "cushion" (really?) is shaped and almost ready. I may rub down the side edges a bit more. I'll walk back into the garage in a couple of days and see what my first thought is. The main cut's were made with a jigsaw, smaller cut's a hacksaw and a sharp knife, and a sanding block finished it off. Every so easy.