Step 1 is to click on the 'User CP' link on the blue bar, upper left corner of this page, and set up an automated signature. It's easy and intuitive to set this up. Use my signature below as a model of the kind of information you should include. It is especially important in a question of this magnitude that a description of your boat year and equipment is provided.

Then be patient, and you will get answers to your question.

Welcome aboard.

I put the system you describe into my boat last summer. Built the locker out of plywood, sealed all the edges with with 3M 4200 as I was building it. Painted it with Gelcoat. drain hole out the bottom is plumbed out the transom, should have gone out under the counter as it sometimes collects a bit of water right near the exit which may prevent gas escaping that way.

I bought a regulator/gauge assembly from West marine and screwed it into the tank. I used a large compression fitting to pass the gas line out of the tank, also from West Marine, and I used a compression fitting designed for passing cables through the deck at the mast to bring the solenoid wiring into the box. An old sail tie is attached inside the locker and used to secure the tank in place.

The box was made to the max size I could fit into the lazarette locker without cutting the frame out. I 'notched' the wood cross beam that is inside the lazarette at the cockpit floor height, so that I could get the locker level and tuck it as far forward in the lazarette as possible.

I bolted the locker in place with three 1/4" bolts through the aft side of the cockpit / front side of the lazarette. Hardest part was getting the lid to mostly seal, weather stripping and a latch that compresses the lid when closed did the trick. My boat surveyor buddy said he would pass it if he saw it on a survey except for maybe the aforementioned drain issue.

The WM regulator assembly leaked propane when I first installed it - simply putting 1/8 of a turn on all the threaded fittings cured that problem and now it holds pressure good.

I sized the locker to hold one of the 10# fiberglass propane tanks + 2 green propane gas bottles that I use for the bbq.

When I got my boat it had been converted to propane but the propane tank was just sitting on a platform in the lazarette where the CNG would have been originally. Not good.