Karen
Sorry about your tortoise.

I would check this section of the forum here
http://www.taxidermy.net/forum/index.php/board,17.0.htmlthis may be more up their ally.
I Do know the scales or scutes on the shell will lift from the actual bone shell over time, so outside may not be the best option.
Perhaps a mold of his shell and a fake bronze casting would look cool. and weather better.
just an Idea.