Personally, I think the 4x4 you have built there would do fine for a couple hatchlings to juvis for a few years, or maybe even to house a single adult, but if you want to have a colony you probably want to pursue the bigger open pen idea, the bigger the better. I'm starting to think that maybe down the road I might try building a cinder block wall with bricks sunk in the ground around the inside bottom edge around my garden pond, if nothing else for a weekend gettaway place for my turtles.
Good luck with your aquisition, if you are not in a big hurry to breed, I hope you consider a CBB hatchling as opposed to a wild caught adopted rescue, not that those turtles don't need a home too, but why not support the people who have spent the time and effort to breed in captivity, and it's fun to watch them grow and mature both physically and behaviorally over the years.
Jeff

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Jeff Benfer
You'll get your regius's to the wall, man!
1.0 pastel Python regius
0.1 mojave Python regius
0.1 normal Python regius
0.2 Terrapene carolina thriunguis
2.3 Terrapene carolina carolina
4.1 Kinosternon baurii
1.1 Malaclemys terrapin terrapin
2.1 Ambystoma tigrinum
0.2 Thamnophis sirtalis parietalis
1.0 anerythristic Thamnophis sirtalis parietalis
1.1 Iowa snow Thamnophis radix
1.1 heterozygous for amelanistic,carmel, and stripe Pantherophis guttatus
0.1 anerythristic motley Pantherophis guttatus