My custom isolation cab has been completed and delivered.
It's designed to look as good as furniture, and contain three 12 inch speakers and a mic for each
When I record in my homestudio, I prefer to do it this way rather than mic up and use headphones

The Outer shell is made from solid Oak, other than the base, and an extra layer of MDF under the oak top.
Then there is hemp insulation, and then an inner mdf box, lined with thick carpet and foam
Intentionally, the "mini-cabs" inside (i.e. the speaker supports) are not coupled to the case, but rest on the carpet.
Since the "mini-cabs" have no back and side, phase cancellation means there is little bass in the room if you use them as a cab, but close-mic'd they are fine

The main problem with the cab has been the weight, and in use ensuring the mic cables don't touch anything since that can add a weird distortion effect.
In use, with a 50w amp full-out, you can hear a little bass escaping, but the output transformer windings on my DZ30 and TW Express make more noise. Certainly quieter than an unplugged strat

Before this I've been using a Randall ISO 12C, which makes too much noise to use in my home studio, but records well - but with one driver you can't match every amp.

Thanks to Ed Lewis for developing my initial plan into something real
Initially I thought to have "mini-cabs" with sides, but Ed realised this was not necessary.

Care is needed to not touch the mic cables on anything that vibrates

All working well - just waiting for a Celestion Gold to put in it

I plan to not move it for a long time! It must weigh more than me. Not cheap to buy that much oak though, so you couls save by using covered MDF
It is a great alternative to an ISO room if you haven't got the space
I now suspect that a smaller cab with one-12"-driver but still with a double-skin would be more practical for most people.
probably with the driver face up resting on a thicklayer or 2 of auralex and/or hemp, and a mic suspended above it. If necessary, the speaker could be constrained that it won't fall if turned upside down but why bother? It would need reconfiguring at installation time anyway, so probably move it when empty

The secrets that few of these I've seen don't include are:
Double skin with insulation between the skins
Don't couple the speaker to the case (like on most of these things).

The Randall ISO12C has a port, to equalise pressure behind and in front of the driver. Why bother? Just don't build the baffle into the case in an airtight way in the first place.

I recommend building one if you need quiet sometimes or want to record into the mix through the monitors