At one time I thought I saw a picture of a man assembling a machine and they are using extruded aluminum sections which are connected to each other by brackets. May be wrong on this, as I can no longer find the picture. If so nothing wrong with that, the people at
buildlog.net - CNC Laser Buildlogs have been doing the same for quite some time and it works well. I am still gathering parts to build one and will also use extruded aluminum for the frame.
The first thing I think FSL will hear is "my machine is out of alignment". Now that will be no problem if the person buying the machine is a DIY type of person and understands this is most likely to happen and does not mind getting everything lined up again after the carrier has managed to knock it around some and the frame of the unit has moved a bit or the laser/mirrors turn out to be a bit out of alignment. A lot of first time users who build the 2x or similar machine say aligning the laser mirrors was not hard but took some time the first time around and you have to understand how to do it.
People thinking all they have to do is plug it in, turn on the water cooling and go for it may be a bit disappointed depending on how the machine was treated in transit.