I used 3/4" plywood here, instead of the usual 1/2", to ensure everything is solid. I don't know how long it'll be until I actually purchase and build the turntable, but I needed to cut the opening now, which forced the decision. I plan an identical 105' table at Durango (also very different than the prototype 65' table that's really there, but oh well).
The benchwork for the roundhouse itself should prove very, er, interesting. It's going to have to a hinged section of benchwork that can swing into place over the basement sink. My wife is VERY gracious with the basement, but she rightly insisted that the sink be fully accessible, and actually I agree. The benchwork will sit about 2' above the sink, but when you have to wash paint brushes or fill large 5 gal buckets, we'll need more than 2' of space. So - the plan is to hinge the roundhouse section itself and let it swing off the sink area, folding back along the Ridgway yard's edge when not in use. This is going to be rather trick, what w/all the alignment issues and mechanical stuff. But - I don't need to build that until I actually want a roundhouse, probably a year or two into the future, at least.