bethgoodtree
There is no fabric that will support such things as shingles and carpet without either stretching or tearing. I suggest you use a solid wood support. Wooden panels for painting and collage is a long-time tradition by many of the art greats, even for simply paint. Your best bet would be to go to some place like Home Depot and ask their experts which type of wood will warp least over time, considering what you plan on doing to it. You might also want to add metal struts across the back, crosswise to help prevent warping and to use for hanging.

Akos
Well your prior answer was almost correct but not quite. You can use tough canvas tarps if you like the service and they take glue well, but stretch it over a piece of plywood for added support. Also don't get a 12 or 15 once one because they are too light. 17oz ones are great. here is a good source for you: or