I've started with 30x30mm rails and 50x50mm sleds. The sleds allow for M8 screw ends to fit inside the square tube nicely. All the rails are 500mm long. The sleds might be too short or I could place the wheels closer to the their edges to prevent rocking. Maybe the 2 on top 1 on bottom wheel arrangement isn't stable enough, if it isn't, more wheels are in order. These are sketches not the final design, the design will transform as the project moves along.