Just wanted to update my initial thread. I decided to go with
hermetically sealed acrylic canisters (the kind used for airtight food
storage). I decided to keep the wooden bases from the original domes
because I already had the butterfly environments glued to them (along
with the butterflies), so removal of all of that would be prohibitive.
The task was then trying to find a suitably sized canister. I couldn't
source them locally, but I was able to get ones close to the size I'd
need online... all I had to do was carefully sand the perimeter of the
wooden bases a bit so they would fit into the canister. To save time, I
used my angle grinder as that part of the base would be hidden by the
canister band anyway (I am using the canisters inverted for display).
The air from the grinder detached two butterflies and I lost antennae on
two others, but a little careful repair work and all back to normal. An
hour later and the new environment is all set up. With the hermetic
seal, I have several moth flakes inside, along with silica gel, and
there was even enough room on the underside to add a humidity meter.
Two hours later and the humidity has dropped significantly and I don't
smell a trace of the moth flakes! And this environment is a lot easier
to service if it ever needs it as there's no gluing that needed done!

Now in all fairness to the original bases with glass, I believe that if
I had sealed the entire base with shellac, I think this would have
worked when used with the silicone sealant. One my two existing unused
domes, the plan will be to file down the bases so that they just fit
into the dome glass; there will then be a round metal base underneath
and the silicone seal will be the glass to the metal base. Someone had
suggested a glass base early on and I may do that if I can find the
right diameter glass, but I'll just use cut out aluminum flashing in a
pinch and no one will see this underneath the original dome base. With
the metal to glass seal with silicone, I'm confident moth gasses won't
get through, but I'll do my first dome just as a test to be sure.

Well, that's the progress that's been made up to now. Really happy with
the acrylic canisters, just don't want to have too many unused glass
domes!