A: How about bolting it to a piece of plywood, large enough so there’s room to hold that board down to the floor with sandbags. Or use a bit more plywood to make a low flat box onto which the lap can be bolted. Before covering over the top, fill it with concrete. You don’t need to bolt it to the floor, in order to have it bolted to a hundred pounds of concrete. Loose rock filling the box would probably do it too.

I’m reminded of the trick some people have used to secure their centrifugal casting machines. The one where you scrounged up a 55-gallon oil drum, put casters on the bottom so it could still roll around, filled it half way with concrete so it weighed a whole lot, then capped the concrete at the halfway level with plywood or something to which you can bolt the machine so that it’s secured, while the upper part of the drum gives you a shield for the machine. Even when the arm isn’t balanced all that properly, done right this can still be totally rock solid. Sheer mass holds it in place. For the lap you don’t need the shield, and could have it lower down, but the basic idea is the same.