What's the point of this exercise? To teach your daughter the virtues of recycling random scrap? Or to make a machine that functions well? I don't see that aluminum as having much rigidity at all. If you wanted to use it as table legs, that might work if you added some bracing. If you've got access to welding equipment and lots of free heavy angle iron, it seems a more promising approach would be to use that to construct your machine. But if you've got a lot of epoxy and granite lying around, you could alternatively build a monolithic bridge to attach rails and screws to. A hybrid approach doesn't seem likely to achieve anything better than either material used separately. Those "tubs" might be useful to put underneath your machine for capturing chips, but it seems unlikely that they'd be much use otherwise. Linear rails need something a lot flatter than that 1/16" sheet steel to mount to, whether or not you've put epoxy-granite on the other side.