If you used some large od high pressure pipe, say in the 1 meter range and had it rated for say 400 psi, that should be plenty adequate.
Or even a propane tank?
But if it was custom made from some surplus tubing, you could put more practical hatches in it.
A sensor array to measure %relative humidity, pressure, temperature could be done, and I am sure a leak proof harness for leads and such could be arranged.
It could sit in a cooling bath, sort of a reverse thermos maybe.
Metal brittleness would have to be addressed, but I bet at these temperatures the allows needed are not too extreme.
A good high uv light source could easily be rated to work at the temps and pressures. A valving system should be fairly straight forward to move CO2 in and out along with an external pumping system to get down to Mars ground level pressures.
With a refrigerant bath and internal overhead heating elements, you could reproduce Mars better than an internal from-the-bottom heating system, I bet.
Should there be a telethon or something?
Ed