I know this is an older post, I'm working on this myself. I rent equipment, and have a bad habit of leaving my white slate attached to the rental BCD. in a pinch, I have used a piece of plastic from the kitchen on a liveaboard, a piece of plastic I found in the closet of a rental condo, and a piece of waterproof paper someone gave me. The paper worked better than expected, but the floppiness sucked. the kitchen plastic wasn't quite white enough, and the condo plastic eventually cracked and was lost on a dive site (I'm a bad person).
I am currently looking for UV stable white PVC that I can drill. I've usually let it dangle, tucking it in the BCD pocket, but I'm thinking wrist mount might be better. I love DIYing!