A copy of the Pso-Rite, the popular self-massage tool.
Purchased some 5/8" dowel from Home Depot, cut to 2x8" sections. 5 minute epoxy or similar to fasten together. I printed my test units at 20% infill and they seem to be study enough, but if you are worried about breaking it then adjust accordingly. I wouldn't go below 15% for this size.