Hmmmm....
When I'm solo diving in a single tank, I carry my deco bottle as a pony bottle (just making sure not to go below the MOD of whatever is in the deco bottle - otherwise, it's not all that helpful).
I have it slung on my left side, attached with clips to my chest and waist D rings. I've seen pony bottles hard-mounted on people's tanks, and there are a few concerns with those. For one thing, it's good to be able to reach the valve, just in case you forget to turn it on when needed. If you have a freeflow on your pony bottle, it's nice to be able to reach the valve to turn the tank off. It seems to me that if entanglement hazards are a worry, having the pony bottle side slung means it would be easier to disentangle it as opposed to if it were mounted to your tank. Also, if needed, you can hand it off to an OOA diver, if that's what you're wanting to do.