Another question. I was planning on using 3" x 2" x 1/4" Aluminium box for mounting the rails on (X & Y axis) but noticed that others have used steel in their builds. I was always under the impression that steel was not straight enough? I'd prefer to use steel as it's cheaper and I can weld the frame up and I need 3 lengths of it for the existing frame so wouldn't have as much waste.