I'm happy to say that the top is done and measures about 55" x 58"
The mitred corners don't work amazingly with clamshells - and they don't match up too well but I kind of like how it looks despite that. I am really pleased overall, though I may not attempt clam shells for a while. I had bought the sizzix die to speed up the process and ended up taking 6 weeks!
I need to find something to back it with now...