Here's how we did it on our previous boat... We made SS tubing hoops that the bottom ring of the tank fit into, attached to the pulpit uprights, and dropped the tanks in place then bungeed them secure. The line
for the stove
lead through the transom via a gland fitting after a solenoid valve.
Don't underestimate the convenience, (and hence, increased safety) of having the remotely switched solenoid. It's a much quicker, easier way to kill the propane source should something go terribly wrong below - and as it's so convenient human nature say's you'll use it more appropriately.