Pleurothallis allenii potting advice needed

[junebug's orchids]

Next on my list of plants to pot is my Pleuro, which is the reason I decided to take the dive into Orchids to begin with

I have this, which I'd like to use as a terrarium, as the room I'm keeping these guys in has ambient humidity between 30-50% and I know pleuros like it moist.



What is it? It's a betta bubble (disclaimer - if you have a betta fish, please don't put it in one of these. they are terrible for pet fish).

This one is about 23cm diameter. A tube of chapstick has been included for reference.

My pleuro is blooming size, but will fit physically inside this bubble terrarium. According to my culture sheet this species maxes out at about 6" tall. Mine is getting close to that already.

So my questions: Do I need to upgrade it to a bigger bubble? If so, I do have a larger one, but it currently has a Dendrobium in it. And, what method of potting do you recommend to keep the humidity where it should be (75-90% according to my Baker sheet)?

My tentative plan was a base layer of lava rock, a small piece of bark sitting on top of it, surrounded by very wet moss, and then moistened moss around the pleuro's roots.