Wrist roller for growing (hopefully) massive forearms.  

TPU is necessary for this. It is used not only for palm comfort but also as a gasket between a plastic part and a metal pipe, preventing twisting and increasing contact points between the parts. 

Also, 6 x 4m screws  (10-12 mm length) and nuts. 

Included STEP files. Double-check housings for nuts; I had a problem fitting them in.