It's done, and worked out well. We used four 1" deep foam insulation sheets (the hard stuff, two 24" x 8' sheets cut roughly in half with the ends leftover to fill in the gap behind) and the debt was perfect and the vinyl used to weatherproof windows. He found all the materials at Jerry's.
We used a serrated knife and a thin flexible saw blade to cut the layers and glued them together. It is not real attractive, but will only be there when quilting large pieces. If I was going to keep it there permanently I would use a piece of flocked tablecloth material under the vinyl to hide the pink that I bought at Joanne's.
It was much less expensive than my sew steady extension table, and I have a lot more slippery working space.
Extension "table" created for my Janome.
[ATTACH=CONFIG]194807[/ATTACH]