1/4 is fine, yes my big female, it was holding a plant and she get inbetween it and the side and broke it. Pound for pound reptiles are second only to insects. That is beautiful work. Think on the PVC if you are planning to heat through it. If not ignore that suggestion. I know nothing of Caulker Cay Boas.
Some use the rule of thumb of the digonal measurement should be equile to the snakes lengh as min. I admitadly know next to nothing about boas but it is not likely that a bit generous space would be a problem.