Its official name is "Chuck Hacker's Interchangeable Thumbillow". I know this because I'm reading it off the instruction sheet that came with the package.
I can't tell you how well it works, because after reading the instructions, I decided that velcro strips, supplemented with white tape on the other side, would do a better job with less effort.
If I decide to give it a try, I'll let you know how well it works.
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seadrive
Cogito ergo bowl