Nope not done anything like that....

IMO the best thing to build a frame out of would be Sq section steel. There's no advantage to making a steel and concrete base, or steel and acetal frame so they're accurate and then bolting an aluminium frame to it that's probably not as accurate. Make the whole frame from steel, build in lots of adjustability and shim/file/fill the top rail (or go the self leveling epoxy route) so it's flat and fit the rails (which if supported round will not be level/flat) to that? Round rails are best mounted upright as the bearings aren't as strong on their side. At 1800 or 2000mm long you're into 20 to 25mm ballscrews to stop whipping at high rpm and Nema 34 motors...or rotating ballnut on 16mm ballscrews. Have a look at Kingcreaky's builds for inspiration (I'm sure there are others like Jonathan's first build to look at too but his spring to mind first as large wood routers, especially the last one!!)

If you want to do any concreting, name a nice flat base for the frame to be bolted to.