2 - 1 X 6 " boards, 20 " long
2 - 1 X 1 " boards, 19 " long
3/4 " piece of plywood, 20 " X 12 "
Metal self tapping screws
Piece of vinyl (optional)
Once you have your materials cut to size, determine where in your boat you want to keep the gas tank. For mine one of the aluminium ribs was in the way , so I had to notch out the 1 X 6 " boards. Install the 1 X 6 " boards using the L-brackets 12 " apart. I fastened mine to the base of my middle seat and the aluminium ribs. These boards do not have to be level and probably won't be due to the fact that the boat floor angles down from the side. Once these are in place, screw the 1 X 1 " boards on the inside of the 1 X 6 " board. At this point it is time to level. Mount your first 1 X 1 " board to the inside of the 1 X 6 " board, closest to the side of your boat. Mount it down an inch or two. This will be your levelling point. Take a level from this 1 X 1 " board across to the 1 X 6 " board closest to the middle of your boat. Mark a level line and screw the other 1 X 1 " board on this line. Once this is done the 12 X 20 " piece of plywood should now fit snuggly between the boards. The reason the 1 X 1 " boards are an inch shorter that the 1 X 6 " boards, on my boat anyway, is that the L-bracket was mounted at the height of the 1 X 1 " board.
Now you can leave everything raw or paint it, I would recommend painting it, as it will protect the wood. For mine, I had a scrap piece of red vinyl and used that to cover the plywood. You can now place your gas tank in the deck, it is level and will not move around. For added security, you can attach a couple of straps to hold the tank.
That solved my problem, hope this will help someone else.