Intro: Multimode Wrist Roller
The human forearm has three degrees of freedom, flexion/extension, ulnar/radial deviation, and pronation/supination.
Unlike the standard wrist roller, which only exercises the flexion/extension aspect, this wrist roller design allows you to exercise all three.
The design as pictured is to be slipped over the end of Olympic-style barbells, which have a 2" diameter sleeve.
The cut grooves on the pipe were an experiment; while they are nice, so long as you have chalk to keep your hands dry, they are not really very important.
Construction of this useful implement can be very simple, though some extra effort will produce a more professional result.
1) Chamfer or round the exposed end of the reducing coupling, to make it smoother and prevent any discomfort during use. A radius of about 1/8" seems to be sufficient.
2) Cut a length of 2" diameter PVC pipe about 12-14" long, and glue a 2"-to-3" reducing coupling onto one end using epoxy or PVC solvent.
3) Secure the rope to the pipe. I did this by cutting a groove in the pipe, bending a tie-down loop's shaft so that it matched the curve of the pipe, and epoxying that in place, but any method of attaching the rope to the pipe which doesn't leave metal on the inside surface of the pipe, where it could scratch the barbell, will do.
4) Make it so that the rope can be loaded with plates. I put a chain link in the middle of the rope and a carabiner on one end, but it would be sufficient to tie a large bowline and secure the plates with a crow's foot hitch.
To use, place a barbell at a convenient height, like the uprights on a bench, load the rope with some plates, and slip the pipe over the end of the barbell. The wrist roller can then be used three ways: by gripping the bell and rotating the forearm in the axis of the barbell, by gripping the PVC pipe and twisting overhand or underhand, and by gripping the bell and rotating it as if opening a jar.