Hey,
There is a 1cm gap on top. When I put the whole thing together I leave the lids on and the shelves rest on top. When it’s all finished they come off with a perfect size gap. The back of the rack system is also peg board so air flow comes through the back over the top and out the front.
As far as the plants go I don’t even bother with the Rhacs, they are just too heavy bodied. Not to mention none of them have the proper lighting for plants. For my day geckos though I use banana trees, pothos and snake plants. The banana trees are my fav, grow fast and spread like weeds. If they are large enough they should be able to support an adult crested as will the snake plant. Check out a local nursery, anything that is big, broad leaved and grows fast should work okay.
-Mike