I used 2 inch sections of aluminium tent pole. Duct tape into place to make the 3D corners then the peg goes through the tube and leaves a 2 inch wall all round. Won't last forever but I've used the stuff before and it is pretty robust.
A tip from making lots of polycro tarps/groundsheets;
If you can get woven (http://www.carrierbagshop.co.uk/fibre-woven-tape.html), use that rather than Duct.
For the same weight it's way more resistant to tearing than Duct (you have to cut it rather than tear when taping up) and it doesn't deteriorate as Duct does - I've got 4 yr old tarps where the tape is good as new. On my Duct ones, the tape has gone brittle & failed (think it's related to UV resistance).