I used the sand circle as a platform and left the mount outside, covered in a plastic bag. Water getting in rusted the bearings on my worm screw, making it necessary to take the mount apart to replace them.
The second version had a piece of outdoor carpet with my rolling shelter on top. This totally failed as the wheels would dig into the sand underneath the carpet. I had considered laying concrete but the time and expense, let alone the irreversibility, made this a no go.
This third attempt is so far the best. I bought 30 (so far) 2 foot by 2 foot 90 lb concrete pavers and laid them last weekend in a 3 by 10 pattern. The metal wheels on the shelter bump along slightly, but smoothly enough and needing little real muscle.
PS Now I have to make some kind of folding doors plus an interlock system so that you can't start rolling the shelter around without opening the doors first.
So far cost is around $50 for the cast iron wheels, $70 for the studs and plywood and a whopping $175 for the cedar T & G siding.
Now for doors and a roof.