I am looking at this at the moment. I really want one but don't want to buy a proper one. I had thought of using scaffold and brackets from this place: http://www.scaffoldingsupplies.co.uk/products/tube_clamps.html
I planned to use two long tubes and two shorter ones to form a rectangular base using these.
Then an upright in the centre of the ends using these.
Then a diagonal to brace the upright on one side using these.
Then just need to make the mounts with bits of tube for the rotational members and a drilled clockface and pinion at 30 deg increments for fixing the position.
Not too sure if it will be strong enough because the recesses in the clamps for the tube are quite shallow. They are meant for guard rail so might not be as structurally rigid as I would like them to be. Still, it can be done with clips I guess although I think this would look messier. You can get casters for scaffold too, which are quite expensive. I already have some so was thinking of fixing those to my frame.
I have gone off the idea of making it mobile now though.