Yeah I am the same really, thinking that the framework can be fairly rigid (nothing too over the top) but not seeing many cnc routers in the flesh it's hard to judge how much force is actually created from them. The idea was to try and create some storage for sheets etc under the bed (as workshop space is limited already) but this can be sacrificed by adding some more support struts etc to beef up the frame.
i was worried about keeping the weight down too for some reason? But as its going to be in the same position in the workshop and weight is a key factor for reduction of resonance and improving quality of finish those worries are out of the window.
still unsure whether to go with alu or steel tho...
matt - my gantry design is based on 2x lengths of 50 x 100mm 6mm wall aluminium in a L shape config. And 60 x 60 x 4mm wall alu box for the frame. Based on your machine would you change the material of the gantry?