peter: oversize projects and large shadowboxes. Great stuff since it comes in a variety of thicknesses. And the stuff is GREAT for covering worktables.
We get ours from a local plastics distributor in 48 x 96 sheets then cut as needed. and, as I say, the waste if large enough we use to cover worktables. You can cut on it, use acetone/mineral sprits on it and it doesn't "degrade" or whatever the chemical term is like carboard or other stuff you may be using for work surfaces.
Finally, in the respective thicknesses, it is about as inexpensive as foamboard
I like Coroplast for for framing and for table tops. It is polypropylene -- relatively inert, chemically. It is also softer, lighter, cheaper, and lasts longer than Sintra as a cutting surface, I think.
Sintra is sheet PVC, which is not so good chemically. I've heard that it is less chemically stable than polypropylene, and may outgas in a closed-up frame.