Best commercial cutting guide Iknow of:
http://www.leevalley.com/wood/page.a...at=1,240,45313
Can be used shortened, or at its full 100" length.

Be warned. It is NOT cheap.

Rockler makes one that is similar to what you describe. I've got
one--not the Rockler--out in the shop, which is probably the one you
describe. I use it, but don't like it much. Hard to clamp with the
small levers if you really want to pull it up tight. And it was not
cheap. It's only a 50".