Your custom bracket looks great. Wish I had the skills to build it. I had a similar need and found something that worked for me.
BAL makes the "Hide-A-Spare" and I think it will work similarly. I use a HideASpare to care my 10' Porta-Bote hull and it works great. There are 2 mount plates that get screw mounted to the frame (one on the left frame rail and one on the right frame rail. The frame that will hold the tank mounts to the pivot bracket on one side and when you lift the other end up, their lockable shelf bracket that locks. Harder to describe than just go see this youtube.
BAL-Hide-A-SpareI ordered mine from PPL in Houston and it was $65. Just note that BAL makes different models depending on your trailer's frame type. You can go to the BAL site or call them for help deciding which model is right for you.
