Update,
Built the bases at home. I did use 4 x4's instead of 2 x 4 or 2 x 6 . All pressure treated with galvanized lag bolts and washers, 1/2" dia.
Excavated the two holes, 40" deep . I have them set in the hole, now need to set in the Quickcrete , level and backfill. If I need to counter weight the top hinged section I can deal with that after I have them installed with the nesting box/bat house.
I ended up using all-thread for the two top bolts that will allow me to pivot the 12 foot 4 x 4 down and up. Jam nuts and washers so I can adjust the tension versus carriage bolts.
I feel pretty good about the base build and the depth. The total height from ground level to top of the 4 x4 will be just shy of 14 foot. I was required to have them a minimum of 12ft.
I am under a deadline to get these things in the ground and set up with the nesting box and the bat box. After that I can deal with some details like cutting the tops of the two base 4 x4s at a slight angle to shed water. I also will install a "stop" so the top 4 x 4 and only hinge one way.
I do appreciate the suggestions and did incorporate a few into the build thus far! Thank!