The word "fridge" has been used a lot of times here, but without going all the way.
What you really want *is* a fridge, and then you want to do some serious case-modding.
The first thing to realize about temperature stabilization is that you need to think about both the energy-flow and the time constant.
Energy-flow first: How much electricity do you feed into your circuit? - You'll have to get rid of all of it again with your cooling system.
If you want stable temperature, the smartest thing you can do is to have stable energy-flow.
If you really want to go hard-core with this, you do not have anything which switches on/off, only things which send the same exact amount of energy in or or the other direction to keep the total energy-flow is constant.
Think in terms of running idle current through your calibration resistors also when you do not measure them, think in terms of Class-A amplifiers for anything regulated (ovens around Xtals, Rb's etc.)
Time-constant is a thing people almost always overlook: You can either go high or low.
If you go low time constant, you have as little (thermal) mass as possible in your controlled zone, and you use an aggressive airflow to keep the temperature stable.
This is far more tricky than it sounds, but is nice if you want to open the door and experiment with things. (This is why professional temperature chambers are expensive.)
If you go high time constant, you add (thermal) mass in your controlled zone, to help keep the temperature stable in time, just like you add big capacitors to keep voltages stable in time.
There are many ways to think about that time constant, there actually is something called "Thermal impedance" and you can get far by using Spice to simulate it.
So back to the fridge thing: If you want a cheap large enclosure with a stable temperature, find an old fridge.
Get rid of the compressor but first get somebody to remove the gas properly, we need that Ozone layer.
Now use the cooling circuit piping to circulate water instead, this allows you to remove quite a lot of heat from the interior, if you need to.
(You will want to replace the thin high pressure tubing, you cant get enough water through that.)
You probably want to control the inlet temperature of the water. Start with a variable speed fan on the external radiator and see where that gets you, upgrade to peltier if necessary.
Finally, if you do not plan to open the door all the time, add thermal mass *and* impedance. Some slices of cinderblock works great for that.
And obviously: Drill some holes for the test-wires, but remember copper is a very good thermal conductor.
Other hints: A small camera can save a lot of door-openings if you need to monitor indicator lights.
/Poul-Henning