I use Animal Plastics PVC enclosures, and they ventilate pretty well -- but they have vents on the sides near the front, and the back near the left and right corners. Your particular enclosure may not have vents that are allowing any air to flow through, so that could be part of the issue.
It has vents on the back at the top but that’s it. Seems like your set up would allow for better cross flow with the air.