Very nice! If you are planning a lid and a light, then make sure it is one that doesn't get warm... If it does get warm and it's closed the glass will fog...
What might be an option is to DIY it with a number of LOW power LEDs that do not get warm and place the controller outside of the container.
And if it will have a tight lid, then watering won't be much of a problem. I guess you are mainly going for epiphytes? These can live and survive in high air humidity pretty good... If it's a relatively closed environment it can't dry out. Then it's similar to a bottle garden and rather self-sustaining without the need for a lot of watering.
Regarding mosses, you can scratch them from the pavement or collect them in nature. You will be in for surprises... I regularly visit a terrarium shop near my place to look at the plants they offer. Also, mainly epiphytes and I'm always amazed about the beautiful variety of mosses growing to the substrate the plants are tied to. I did ask the shop owner about it and he said he doesn't culture mosses. And all came on their own devices... And I've seen mosses that I've never seen before, but it can only mean we must have the spores flying around also ending up in the terrarium shops greenhouse. Then land and germinate there when conditions favour it. Then we must have this whole variety of mosses flying around all over the place and a lot probably dormant laying around till the correct circumstances are offered to germinate.
I do grow and collect a lot of mosses from nature also simply scratch them from the pavement. Put it in jars and after a while see varieties and grow forms in it that I didn't intentionally collect.
🙂
What you also might like to put in there are the smaller species of climbing ferns such as the Pyrrosia sp. and the tiny Barbosella sp. Orchids or the Bulbophyllum sp. And there is quite a nice variety of smaller terrestrial Utricularia sp. available that would love it in there.
Have a peek here for inspiration and ideas. He has a rather extensive terrarium plant database. And if available he always has top quality plants.