I think that there is no need to use esoteric alloys with the machining problems and costs involved. If you use the thermal coefficient of expansion for a metal; calculate the change in dimension of your longest piece say from 20 degrees celsius to 35 degrees. There will be such a small dimensional change that it probably is not a factor. I had an enlarger with a 12' aluminum column (bed) and did not need to take temperature into account between summer and winter even with precision work.
My suggestion, once you get beyond the initial design experiments, would be to use a vacuum formed plastic housing.
A determined tinkerer can do this in a kitchen.
Google vacuum forming and check out a few you-tube links.