First off, don't think plaster of Paris. Setting is a chemical reaction
and it is exothermic. In a big block, that will get hot enough to burn
you through your speedos ! That's probably true of anything that sets
quickly.

A better idea is not to use anything that has to set. Just sit on a bag
of wet sand and shuffle round until comfy. You can now take a cast of that,
or just use a profile gauge to take lines off it. In the latter case, you
can repeat the process several times (perhaps in different clothing ...)
and take an average of the profiles you measure. The other thing that would
work is bean-bag filling, in a bag from which you can suck all the air.
This is how they immobilise spinal injury cases - a careful lift onto a
pellet-filled bag, then pump the air out so it "sets" to the shape of the
casualty. The process is easily reversible (for easy casualty extraction,
or repeated bum prints...)

I have to say that I just buy a foam seat insert from Knoydart in Penrith
and put it directly on the floor of the boat. This has the advantage that
you can take it out to sit on for lunch :-) (and the disadvantage that it
might blow away, so a tether of some sort (velcro?) might be in order, but
I never have got round to that).

Andy