How do you get the humidity down?

[Malum Argenteum]

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.

Yes, that might be a little part of your issue. I'm not sure your RH is actually too high, though, so I'd recommend being cautious about making any changes.

One thing increasing ventilation would do is make the 'warm side -- cool side' gradient more pronounced. Since you're using heat tape (not ideal) in a room that's apparently not too warm (I'm guessing this from the condensation on the sides), I think more ventilation would make an RHP a near-necessity. This isn't necessarily a bad thing, but it may be more than you're bargaining for.