Buy a standard wrist roller if you want, but a homemade version of PVC pipe (1.5 to 2 inches is perfect) with climbing webbing attached is a cheap and amazing solution.
Check our power-rack version that we made out of aluminum tubing. The beauty of it in a rack is that you can truly test the grip without the shoulders giving out first.
For this mechanical drop set, you'll start with the hardest exercise and, as you fatigue, move to the easiest. Works with a bar and a supinated grip too.