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.