+1 on the Tyvek as a ground cloth.
My only concern with Tyvek for this application is finding a piece large enough for the foot print. Seems like most of the time I have seen it is in rolls that are 4' or maybe 6' wide.
As was stated you can/could sew it to make the correct size.
It is extremely light for its strength and compacts nicely for packing.
I have made several for smaller tents.
