I use the Christmas Tree Box for mine too. I had her in bigger when I first got her and she hated it (4'X2'X18"). She was very stressed out. I moved her down to this and have had her in it for three years. She has been a happy camper. She is 6' 3" and about 17lbs. I think she just likes being snug. I have a heavy large corner bowl and a cat litter box with the corner cut and sanded off for a hide. Its not the fancy set up I was first gong for but I'll take a happy snake over the alternative. I have the tub set up with 11" heat tape and on a VE-200.
Luck to you
PS: Mine was a rescue. Besides being a medical mess she was kept in a 15 gall fish tank (at 6 feet) and there was another in with her (he died). After being contained like that I think my 4 foot enclosure was hard to adjust too and could just be a quirk of hers. On the other hand I have met a few others that have had theirs like it snug too.