If you do a search on this forum you will see tons of examples of peoples set-ups and there are several posters with similar set ups to the one you have proposed, so hopefully they will post examples here as well, but this site isn't as active right now as it is in the summer. Generally cedar and redwood are not advised for use with reptiles, due to the toxins that naturally occur in those aromatic woods, which is probably more important in an indoor inclosure, not sure that would have a big impact when just used as an outer wall outside, but arsenic free treated pine (which most treated pine posts are now arsenic free), would be the safest bet and should last a very long time, you can find treated pine post at big box hardware or lumber stores, like Lowes and Menards, ect. how deep you place them probably depends a lot on your soil composition and how heavy and dense it is ie, if clay not as deep as sandy or loam, and think Tess Cook recomends in her book; "Box Turtles", and on her web site that they be at least several boxies high 16-20" and having them lean slightly inward and/or having an inward facing cap on top of the wall, preventing climbing escapes, which make perfect sense if you think about it, maybe she will chime in as she has a lot of experience with that type of set-up, as do others, I do not, but just wanted to help get you started looking, researching, and thinking the right direction.
-----
Jeff Benfer
1.0 cinnamon pastel Python regius
1.1 pastel Python regius
1.1 mojave Python regius
0.3 normal Python regius
1.3 Terrapene carolina thriunguis
2.3 Terrapene carolina carolina
4.1 Kinosternon baurii
1.1 Malaclemys terrapin terrapin
2.2 double het albino and anerythristicThamnophis sirtalis parietalis
1.0 anerythristic Thamnophis sirtalis parietalis
2.3 Iowa snow Thamnophis radix
0.2 het Christmas albino Thamnophis radix
1.1 double het cherry erythristic, albino Thamnophis sirtalis sirtalis
1.1 melanistic Thamnophis sirtalis sirtalis
2.0 66% het snow Thamnophis sirtalis sirtalis
1.2 flame Thamnophis sirtalis sirtalis
1.1 triple heterozygous for amelanistic,carmel, and stripe Pantherophis guttatus
0.1 anerythristic motley Pantherophis guttatus
0.1 butter p.h. stripe Pantherophis guttatus
0.1 carmel stripe p.h. amel Pantherophis guttatus
0.1 amelanistic p.h. carmel,stripe Pantherophis guttatus