and came up with the following solutions:
Ditch the MDF sides and go with an aluminum base and gantry.
Use 1.5" Schedule 80 aluminum 6061-t6 pipe (1.9" OD 1.5" ID).
Drill and tap rail for 0.25" fine screw every 6 inches.
Rail is housed in 2"x4"x0.25" aluminum C channel.
Drill the bottom and the side of the C channel for support.
Use a double nut assembly for adjustment.
See Pics:
And
Though I think square aluminum tubing would be easier to drill and tap.