I would like to scratch build the canaas top on a horch but folded at the back of the truck. Like a converatable. Any ideas how to do this?

Padrino
My first thought is to make a wire frame much the same as the 1/1 scale. Then cover it with very fine silk.
Another thought is fine cheese cloth, or the tissue paper that you use to wrap gifts. Its somewhat stronger and more plyable than fassial tissue.

I've seen a number of posts where builders use putty to mold and shape softskin covers. Since all the canvas is in one tight location that might be an easy way to go.

Just my 2 cents.

Padrino,
One of the very first posts in this forum deals with this topic : making canvas .
As far as I'm concerned, there's only one good way, and that's the tissue paper & white glue trick. Read all about it in that forum, and try it out on a dummy model or any other shape before you do it on your model. You'll just LOVE the result.
I'm interested in finding out how it went !!!
Good luck,
Jan