The X axis is taking shape, I used an H shaped layout for the carriage, with drywall screws and skate bearings. There is no spring tensioner, the placement of the bearings press slightly against the rails, and there is enough give in the bed to fit it in without any play. This was accomplished by using washers as spacers between the end of the cross piece. With the two screws, after getting close to the right number of washers, the exact alignment is done by tightening or loosening the screws a small amount, this results in the end piece twisting, with a bit of finesse eventually it ends up being exactly right. The vertical alignment is simple by comparison, just use a single screw on the other end and tighten it, it should be no problem to force all four bearing wheels onto the track with the wood friction keeping the joint tight.
I also trimmed some plastic from the bed near the end to make clearance for the motor to be mounted to a threaded rod. There is a small tendency of the carriage to ride up out of it's slot, but when the rod is attached that will act to keep it from riding up. The profile of the rail should not matter much. The same setup should work fine with metal rod, pipe, angle iron, etc. Any profile where you have a straight inner surface, and a straight top surface.