" Do they stay in place pretty well (as in they don't slide down toward your hand or twist around)?" Yes, they stay in place very well. " Where did you get your black plastic barrel?" You can buy them new or used. A new barrel is about $85 I believe, but used for much less. I work in an industrial area where they are often just thrown away. Just be careful to make sure there is not anything caustic in them. " And do you think something similar could be made for the upper arm?" Absolutely. We used to make full suits for "hidden armor" so that it looked like you were just wearing clothing. Very versatile.!