I think the solution is simple: You need an enclosure that has metal walls, preferably a one-piece thing so it would not leak, preferably with ribs on the inside and on the outside for better heat transfer. This would dissipate enough heat to be safe without any outside pumps.
Bingo. You read my mind. I have a friend who likes to weld. He's going to teach me how (I've always wanted to learn), and the teaching project is going to be an aluminum oil-proof case with fins on inside and out. Technically, I won't be welding -- I'll be "brazing", which is essentially soldering. Aluminum is hard to braze, but we've learned of good aluminum brazing products.
If I could find a good source, I'd do it in copper (twice as conductive as aluminum). Could be pricey though.
Using copper will be a waste of money, aluminium will be just as good.
Eventually (might take a while) the oil will reach a certain temperature when the heat output of the PC equals the heat emitted to ambient (by means of the wall containing the oil).
To keep it a bit simpler than it really is, assume all of the oil has the same temperature, and the wall of whatever material will act as a temperature bridge between the oil temperature and ambient temperature (the temperature inside the wall will drop slightly from the oil temperature where it touches the oil, to ambient temperature where it touches ambient air).
[Note, the temperature of the wall where it touches ambient is not
the same temperature as ambient itself - the wall acts as as a radiator at this point, this is where the heat will be extracted from the system]:
In this graph, T1 - Tw1 - Tw2 will be virtually the same, and the big drop happens at T2.
Thermal conductivity of
copper: 401 W/mK
aluminium: 237 W/mK
Means a 4 mm thick copper wall is as thermally conductive as a 2.4 mm thick aluminium wall.
The impact on the oil temperature is negligable, though.
If the PC has a power output of 400 W (which is pretty high end), and the walls are a total of 0.6 m2, a 4 mm copper wall means a delta T of 0.06 Â°C. Aluminium will mean a delta T of 0.11 Â°C.
The difference is practically immeasurable.
To put things into perspective, "plastic" (pyrex glass) has a thermal conductivity of 1.0 W/mK. It is a very bad thermal conductor, the result is a delta T of 26.7 Â°C
Using metal walls is a good idea but the differences in temperature will be very small when compared to plastic walls.
I even wonder if adding fins to the inside
will be a good choice, you will be increasing the area but the thickness of the wall will increase as well, maybe doing more harm than good. In all cases, a wall which means a barrier of less than 1 Â°C is nothing to be concerned about.
On the outside, where it touches ambient air, you will want to be the area as big as possible, because air is a bad heat conductor.