Thanks for this! I have been wanting to mount a Seahorse case (Pelican knockoff, but with the option of keyed latches) as a tank bag, but couldn't find a universal mount... Though I didn't look too terribly hard.
I just ordered the Seahorse case and ring lock mount for it.
EDIT: I did a Google search and was reading through a thread here where someone mounted a Pelican case to the tank with this ring lock mount... and then noticed that I posted in that thread saying it looked like a good idea... Looks like I did find the piece I was looking for. I must've been drinking or something that night!