I made a pvc drum to fit inside my rubber lined Frankford arsenal tumbler. Three inch sch 40 p.vc pipe 15 1/4" lg, use 3" plastic test caps to seal ends of pvc pipe. Turn the screw on caps backwards [ the caps will protrude outside of the tumbler body instead of inside] and use a round 1/8 " silicone table protector that you sit a glass on to give some adjustment when tightening the caps down. Have only tumbled my ball media to test it but seems to work fine. Am looking for a used tread mill with adjustable speed control to make a bigger volume tumbler. Using sp 45 acp brass filled with hard lead now but will probably try small non magnetic ss bar stock cut in short pieces or maybe brass round stock for my new mill media in my larger tread mill ball mill. Thinking 6" sch 40 pvc pipe will work.