Do you have a link to those mu-metal-boxes?
Here you go:
https://www.ebay.com/itm/Mu-metal-nickel-iron-alloys-shielding-case-for-transformers/282939791744?hash=item41e0869980:g:S4cAAOSwt51auf2gRather small since they are made for transformers; not a box in the generic sense. I like the pre-made holes since I've read that Mu-metal may need re-annealing after being formed or machined.
The aluminiumbox doesnt need to be thermally coupled to the LTZ-board as no component generates so much heat, that it needs to be cooled. Also you generally want a good thermal insulation of the LTZ-board to the case and mechanical flexibility of the board to avoid strain-induced offset-voltage.
I suggest following AN159 on shielding box construction, for generally good shielding: https://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf Page 7.
I guess I was somewhat unclear regarding the purpose of my aluminium "box" - it is to be a heated oven. I'm planning to custom-machine an aluminium slab to fully encapsulate a voltage reference, a heater of some sorts (like the '723-based version described in another thread) and possibly a step-up/-down board to get a specific voltage like 5V or 10V as output. Thus, my goal is to maximize thermal transfer to each component and have a large, hopefully isothermal, mass containing all the electronics. In practice, it will be at least three "slabs" that make up the whole enclosure, I'm thinking a top part with MOSFET heaters, a middle part with heater control and Vref, and a bottom part with MOSFET heaters again. I'm aware that a fully custom encapsulation will mean that every dimension, specifically for resistors, will be unique so there will be a need to machine new parts if I change from WW to BMF, for example. Circulating air is supposedly a bad thing, so I'm planning on very narrow encapsulation of the components (say, 1mm). My question is if it's good to leave it like that - with an air gap surrounding each component - or try to actually make thermal contact using some method?
I had some thoughts of other materials, like Ag or Cu, but I've read that machining is very difficult and would require casting. Since I've worked with aluminium before with good results I'll stick with that - I have a couple of CNC machines so I won't have to rely on my unsteady hands.
Hopefully, I've explained what I'm aiming for in understandable terms.