I like your idea of using a bolt to pack the sand. Very clever indeed.
I'm not trying to toot my own horn here just offering an idea for building a custom saddle.
In an old build thread I detailed building a seat frame. It was formed from 3/8" bar stock with 1/8" X 2" flat stock cross bracing. A local upholstery supply house carries what they call 'motorcycle foam'. It is very high density stuff. Using 2" thickness gives a soft but firm ride and will retain the shape you form into it.
The seat build starts on page three and there is some photos of the upholstery job on page ten. http://motorbicycling.com/showthread.php?t=44276&page=3