Prototype. 

Print this with Prusament polycarbonate carbon fiber filament to achieve require stiffness. You will need to anneal this carriage in your oven at 285*F for two hours to acheive 130*C heat resistance which is higher than the print bed temperature to prevent warping while in service.

100% infill, 0.18mm layer height. 180g of filament (original MK3 carriage 300g)

Use 30 minute epoxy or JB weld to bond the halves together with 4x M3 25mm bolts.