I, too, went with the 5x5 Home Depot fence post sleeve. For my application, I fabricated my mounting brackets from 1"x1/8" aluminum stock. The 5x5 caps weren't available, so I cut a couple out of some 1x6 pressure treated I had laying around. I still gotta get by a Lowe's for the gutter insert. I like the idea about the locking hasps, as well as using the gutter insert for a hose support at the CG.
5x5 post sleeve
here
Heating and bending the aluminum:

Two brackets. They weren't bent with CNC precision, but they're pretty good for a mouth-breathin' water head and close enough:

I hit the brackets with a polishing wheel before mounting. The acorn nuts and bolts are stainless, as are the screw-eyes I used to secure the end plugs:



I had to have something to plug the ends, so we could hit the road:

Moderator edit to re-size pictures to forum limit of 640px maximum width.