Thanks for the help guys... Here's the final final...

http://www.ejearchive.com/temp/tumbler2.jpg

As it turns out, it doesn't need to have any grip material at all on
the edges. I roughed it up with sanding and that was good enough. A
normal belt was also good enough.

Only thing now is to sort out the inside. It seems to work without any
liner, but it's very noisy. I tried using that anti slip shelving
stuff taped in a pipe shape, but without gluing it, I think it was
just slipping around. Whatever material is used, it will have to be
glued in well. It's obviously a very hostile environment to make any
glue stay. Ideally what would be great is to have a hatch that opens
along the whole length. It's kind of a pain having to dump the stuff
out in a bucket, then put the media back in each time.

Dave