Nice work! Very cool to see this actualized. This is very similar to what I was thinking about 10 years ago. Hoping you didn't suffer that unfortunate illness too badly and are free of any lingering effects moving forward! A few comments-
Holes were made near centrally in the base for the mics and a quick test revealed the surfaces were too reflective so the interior surfaces were covered with 2.5mm fun foam (it's what I had left over from another job).
I'll start with this, as it makes for a logical continuation of the discussion of boundary-layer-effect in the preceding posts above. The walls of your arrangement serve as both
baffles that modify the directional pickup pattern of each microphone, and as
acoustically reflective boundary surfaces which increase sensitivity and the ratio of direct to reverberant pickup. Both of these effects will come into play above a particular frequency which is determined by the dimensions of the baffles and the area of the boundaries which they form. Below that frequency, both the baffling and boundary effects are minimal. Ok, so what are the implications of this?
A basic one is that boundary-mounting benefits from mounting the microphone as close as possible to a hard, acoustically reflective surface. A baffle does not require the microphone to be close to it as long as the surface of the baffle is soft and acoustically absorbent within the frequency range in question.
Taken to the logical extremes, this means the microphones should either be mounted as deep into the corners as possible, or the baffles should be as acoustically absorbent as possible. There are arguments for each approach.
To optimize the boundary effect, the surface of the boundary should be acoustically reflective (hard), and the microphone diaphragm should be mounted as close as possible to the boundary surface - flush embedded into it if possible. It is the flush-to-the-surface mounting which avoids the problem of reflections off the boundary interfering with the sound reaching the microphone directly. In your case there is not just a single boundary surface but three acting together to affect each microphone - two vertical wall boundaries and a floor boundary. Ideally, the microphone should be mounted with its diaphragm as deep into the vertex as possible (buried as far as possible in the corner facing outward).
It looks like you can easily get closer to that kind of arrangement without much trouble. In your photo above it appears each microphone cable passes through a hole in the "floor panel" which is about the same diameter as the microphone itself and is positioned well back into the corner. Try installing each microphone so that it barely protrudes through that hole, with its diaphragm facing directly upward, leaving most of the microphone body extending down below the "floor panel". This will get the microphone element much closer to the vertex and reduce combfiltering from reflections off the boundaries. You may also notice increased high-frequency response as the microphone is moved closer to the vertex, which is the boundary effect working to a higher frequency given the closer-to-flush placement in the corner. It might be that high-frequency boost nicely offsets the attenuation imparted by the squirrel fur. If so, great. If not you can correct for it with EQ, or play around with using foam and/or fur on the boundary surfaces. If the microphone is placed flush enough to avoid comb-filtering reflections, making the boundary surfaces less acoustically reflective and more acoustically absorbent will reduce the sensitivity boost effect in the range in which the boundary effect is occuring.
Alternately, if the microphones are not flush mounted, you can make each baffle surface as acoustically absorbent as possible and rely on just baffle effect, like a multichannel Jecklin disk baffle. It will take a bit more than thin foam to effectively achieve this. When I was playing around with Jecklin disk baffles, I made a bunch of tests to determine how much absorption I needed to reduce angle-of-incidence reflection across the frequency range in which the baffles were effective. I initially thought I'd just use a layer of foam and fur, but ended up with a baffle core made from corrugated cardboard supporting a layer of cut-pile carpet, atop which I placed a layer of felt (further covered by fur) on each side. In other words, it took a lot more than I first imagined it would to effectively damp reflections off a surface in close proximity to the microphone.