Diffusion cloth is thicker than gel and, I would think, not as good for what you want. With a bit of care, you should be able to join two rolls of gel together without the seam showing. Bearing in mind that you'll want to end up with a pure black shape on a pure white background, with no greys at all, a seam which is just about visible can be "lost" by increasing the contrast in post.

There is a back projection gel available (which is identical to #216) and that is produced in 10 ft widths but is very expensive and, according to my missus who has used it for back-projection, needs more ooomph than 216.

If I were you, I'd get a yard or so of gel and do a bit of experimenting to see if it's what you need. Otherwise you might have to go down the chromakey (green/bluescreen) route which needs a lot, lot more skill than a back-projected shadow screen.