This is a problem lots of large builds have to deal with. One solution is to make some dams on top of the horizontal members, and fill them with self-leveling epoxy. If you do it right, you end up with two co-planar surfaces that are flat and level, through which you can bolt your rails. Here's a thread from someone who tried it - he struggled, but maybe you can learn from his mistakes: https://www.cnczone.com/forums/cnc-w...gineering.html