You might also consider stripping your bed, the bigger the better, and use it. You could pin the backing right into the mattress and then layer on the batting and top. There might be some overhang. In this case, I would probably work from one side toward the center, pinning, etc. The roll the completed edge for 15" or so, and then pull/lift the unpinned side onto the mattress.
When I lived in DC, I had a large padded table (5' x 9')which I called my drapery table wherein I made everything custom for the home for other folks, of course. It is up on horses. I still have this "big" table and many of my quilting friends call and ask to use it to baste a quilt. When two or three of us get together, we can baste a quilt rather quickly and enjoy the time just being in each others company.
I live in Central PA. Anyone want to schedule the "big table"? You are all welcome. The more the merrier.
Suzanne