Ventilation in terrarium with computer fan?

[Fishkeeper]

I have a 12x12x20 Zilla terrarium, that I'm trying to rig for micro-mini orchids that like to be lightly and evenly moist. I'm having trouble keeping humidity nice and high without soaking the background too much, so I'm thinking about closing off some of the external ventilation. I'd like some thoughts.

Right now, it has a ventilation grate maybe 3/4" of an inch tall, running across the lower front of the tank. There's also a mesh strip about 1" wide, across the back of the lid, and a few slim gaps around the plastic covering the rest of the lid. A 3.5" computer fan is fastened to the underside of the ceiling, with some clearance above it, pointing down. It runs constantly, which is probably why the humidity keeps escaping. I don't want to shut it off, so I'm thinking, instead, I should cover some of the slit on the lid.

I don't suppose anyone else has a terrarium of roughly this size, with a constantly-running computer fan?