Hi
Wish I could help, Though I grow a lot of succulents and epiphytic cacti have NEVER been able to grow them in a terrarium lol. They rot.
How about just a thin layer of sand on top?? gary

Yes. I'll put layer of sand all around the bottom of the terrarium; pour in my succulent/catcti mix; plant everything; and cover with a thin layer of sand. So, the whole thing will look like it was planted in 100% sand.

Hi
Sounds like a plan !! Have you thought of a very thin false bottom?? Would increase the drainage. gary

There's a reason you haven't been able to find much if anything on cacti terrs, Jeff -- they generally end in disaster. It tends to be extremely difficult to provide them with the high light they need but not "cook" them in the process & to provide them with the arid conditions they desire. As Gary mentioned, they easily succumb to bacterial or fungal diseases if the humidity is not low. Terr by nature tend to keep humidity in. Hopefully the air circulation you mentioned you will provide will do the trick in that area.

Overwatering, ever a danger with most terrs, is an even greater issue with a desert terr. I think Gary's idea of a false bottom is probably something to consider. In that case, you won't want any of you media to actually reach into the false bottom and wick up any moisture that drains there.

I look foreward to seeing your set up! Wishing you the best :)

Thanks for all of the advice. As I mentioned in my initial post, the terrarium has a very large opening for air circulation. The plants have been thriving for weeks as the circulation never allows the temperature to get much higher than the air outside the terrarium.