Your knife can double as a tank banger.
Get wrist slate.
I use a large stainless steel nut (M12 is good), on a loop of bungee cord that has been threaded through the nut and tied with a reef (square) knot. a DIY diver's tank banger. The steel nut produces a better sound tone than plastic.
I keep it below the lowest cam band on the tank.
Sort of like this.
![IMG_0691.JPG IMG_0691.JPG](https://scubaboard.com/community/data/attachments/389/389517-d4044a2c36f665dd8734e78a8f79995a.jpg)