I've been playing around with a similar set-up. The problem I'm having is I added a fill spout to the top with a screw-in cap. When I pressurize it, water is forced out of the screw-in cap around the threads. I've tried a couple of different thread tapes, but it still leaks. And, I'm only adding around 10PSI. Looking at your set-up, I think I'll try replacing the screw-in cap, or maybe glue it in with ABS glue and add the hose bib for filling.
The other problem I ran into was using the pipe clamps, as you did. It doesn't seem to take much "off-roading" to cause the tank to shift when full of water. I'm gonna give one of these a try:
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