What to use to make a mold?

At 14:35 26 March 2009, The Real Doctor wrote:

He has said elsewhere, I think, that this is for placing under him.
And he could fasten the cushion down.

I would be worried about the design strength of any fastenings to flimsy
seat pans during high g impact decelerations.

In general yes, but surely it depends entirely on the glider's weight,
balance and limits?

If it's for C of G positioning then the further forward the less mass
needed. You may exceed the all up weight limit with sufficient mass at
the seat lever arm for CG positioning; also I'd not be particularly happy
about speed performance ballast I couldn't get rid of in the air if I
ended up low and scratchy!