I usually carry mine slung, with the hoses wrapped in bungees, much like the picture in post #4. I normally clip it to a shoulder strap on the front of my rig, but I have clipped it elsewhere when I planed to be doing something that involved me laying on the bottom, and a shoulder slung rig would have been in the way.
I have tried a few different rigid primary tank mounts over the years, but none of them gave me the easy access I wanted nor the easy ability to hand off the pony in the event that someone else needed it. I find that an AL40 is a bit much to carry shoulder slung, but doable if I really need it. An AL30 is still a bit much, but not as terrible. An AL19 is not so bad to carry. Anything smaller than that is easy. It's been years since I've used a hard mount on the back. I really prefer to carry the pony slung.
When I did use a rigid back mount, I found it least cumbersome to mount the pony with the valve facing down & then use a long hose to the second stage. I would normally clip the second stage near the top of my shoulder strap.
If you are going to fabricate a custom bracket, consider machinable plastics, such as Delrin, as a material rather than metals. They are easy to work with, fairly strong, not so heavy, pretty much neutrally buoyant, & there is zero concern for corrosion. I like a neutrally buoyant pony rig. That way, I don't need to change my weighting if I decide to carry it or not carry it at the last minute.
I have tried a few different rigid primary tank mounts over the years, but none of them gave me the easy access I wanted nor the easy ability to hand off the pony in the event that someone else needed it. I find that an AL40 is a bit much to carry shoulder slung, but doable if I really need it. An AL30 is still a bit much, but not as terrible. An AL19 is not so bad to carry. Anything smaller than that is easy. It's been years since I've used a hard mount on the back. I really prefer to carry the pony slung.
When I did use a rigid back mount, I found it least cumbersome to mount the pony with the valve facing down & then use a long hose to the second stage. I would normally clip the second stage near the top of my shoulder strap.
If you are going to fabricate a custom bracket, consider machinable plastics, such as Delrin, as a material rather than metals. They are easy to work with, fairly strong, not so heavy, pretty much neutrally buoyant, & there is zero concern for corrosion. I like a neutrally buoyant pony rig. That way, I don't need to change my weighting if I decide to carry it or not carry it at the last minute.