I'd had the same idea of packing the base up for shallow stuff. I may make up a second tall base from MDF using a torsion box design.

The challenge is to reduce flexure to the minimum, both by designing in stiffness and by restricting the offset distance of the cutter from the centre of each beam.

My Y axis beams are 2" square, 10g, alloy box sections, with 16mm fully supported SBR type rails fitted to the top of each. These beams are on 6" centres horizontally, so I have a gap of 4" between them. The Z axis will (I hope!) fit into this space, so that the cutter can retract back between the rails for maximum usable operating depth. Including the height of the rails, my Y axis beams are about 4.8" tall, so I'm hoping that they should remain stiff enough in the vertical axis to not noticeably deflect under the weight of the carriage and spindle.

I'm mounting the rails on top of 8" long 4" x 2" channel sections, mounted with the short faces at top and bottom, bolted directly to the carriages of the X axis. The X axis rails are also 16mm fully supported SBR ones, bolted to the top of another bit of 4" x 2" channel mounted on edge, with the bed bolted beneath. This gives me a nominal Z axis working height of about 248mm, although I can gain a bit more, if needed, by lowering the table a bit more.

Jeremy