So, while installing a ridged climbing tube between the upper and lower floors of the habitat tower, I learned a hard lesson about sealant quality; Upon reassembling the tower I found I had run out of marine grade silicone sealant, so I used something purporting to be just as good, a transparent goo that I immediately discovered was complete crap at waterproofing. Upon test-submerging the reassembled habitat (without the hamsters inside obviously) it slowly began taking on water in the recently reassembled section. I took it out, checked for leaks, added more sealant, waited another day for it to cure, but no dice. Leaks persisted, to the point that the habitat is now in pieces again awaiting proper sealant prior to reassembly.
However,in that time I happened across a few parts that fit perfectly together by pure coincidence. The domed lid of a frappucino (with the large hole for the straw) snaps onto the bottom part of a mini-DVD spindle, which serves as the floor. It does so very tightly, water does not get in. As for the large hole in the top of the dome, the small clear bubble from a 25 cent toy capsule happens to just barely fit when shoved through from below, such that it forms a sort of compound dome that is rigidly watertight without any glue.
This left me with a floor, and a compound dome. I drilled a hole in the floor piece and dremeled it out until it was large enough to stick a ridged climbing tube through and I'm now waiting for the marine loctite epoxy to cure. When the loft section is done, I will drill and then dremel out a hole in the cieling of the main living module and install the loft as pictured here:
You'll notice there's also a water bottle I've already installed. This is because I can't be sure whether or not permitting outside water in via the drinking tube, even through a filter, works properly. The Mk.III habitat used this mechanism but I could not be inside to test it while submerged obviously and the hamsters had a dish of water as backup in case it didn't, or in case they refused to drink even filtered lake water.
My main concern is that the internal overpressure would continually push air out *through* the drinking tube, permitting no water to get in through it. So even though the water intake mechanism would hypothetically offer unlimited drinking water, I've opted for a finite water bottle that I know for sure will work because it does not connect to the outside water and therefore won't permit any kind of pressure related shenanigans to occur. Plus it has a little plastic turtle floating in it so I can see when their water supply gets low. :3
Incidentally I have also replaced the kitty litter with planting soil from a garden shop. Bacterial action in said soil will continually process the hamster waste, and with the help of clover I've planted in the soil, it will metabolize it at a rate sufficient to prevent buildup. This means I will soon be able to leave the habitat underwater for as long as the food and water hold out, which (judging by their rate of consumption) could be a month or more. If I can prove to my own satisfaction that the filtered water intake works (by observing it in the test tank) then that could be theoretically extended to many months, assuming I put in an overly generous stockpile of food.
I've learned a valuable lesson about which sealants work and which don't. Now I can be confident that the Mk.2.5 habitat, once fully rebuilt, will not only sustain hamsters much longer than before without surfacing, but also be more reliably watertight than ever.