"I used 2" x 6" and lag bolted the ends together, then levelled everything starting from the highest corner using cut up pieces of angle iron driven in the ground, then screwed into the 2" x 6" when level.( I found some steel shelving uprights with holes already drilled). I have about a 12" difference from high corner to low corner. The total size is 18' x 20'. I then filled the gaps below the 2" x 6" with strips of 1/4" plywood screwed from the inside dropping down to the ground where ever necessary in the low areas to support the tarp. I then packed snow around the outside of everything for extra support."

                   Good job and thanks for the picture, Chris.