I'm about to do the same. I rigged up some wheels because I don't have the cheese for bumpers, but it still rattles the house when I drop it.
I'm basically going to frame up a 6'x8' box using 2x4's. The 4' wide center section will then have extra 2x4 joists (floor supports) spaced 6" on center and then covered with a piece of 3/4" plywood. If I get any shifting or need more support for force transmission, I'll flip it over and fill the gaps with concrete. That should make it plenty heavy. I really hope the concrete won't be necessary (and if it is, I REALLY hope I never have to move it!).
The landing areas are going to have cushioning instead of support so they'll absorb shock instead of transferring it to the floor. I'm going to cut one of my 4'x6' horse stall mats in half and mount them to 2'x6' pieces of 3/4" plywood. These will 'float' in their holes, and I'll fill under them with sheets of carpet pad. That should distribute the load evenly across the carpet pad, so the weight won't just sink in (500lbs over a 4'x6' area is only 2.3 ounces per square inch), but it should absorb most of the energy.
If I store the weight on it, or if the landings ever concave, I'll need to flip them over for storage so they bow upwards.
Might be a little while before I build it (but not a long while). I'll take build pics if you want.