ok, first attempt using sketchup 8

[JAZZCNC]

I like the idea of using the RSJ, less for me to do. I assume i wouldn't need to box this in and it can be left as is, would it need support or stiffeners at each end.

Here's how I'd go about it.! . . .Miniumum welding and very strong. I've drawn it with welded corner braces but this could be built without any welding what so ever just drill and bolt if you used 19mm plate for the corner brackets.
Personally I'd weld 5mm plates in the corners and proably a few extra braces across it's length round the back side to just help reduce resonance but again these could easily be bolted, just weldings quicker.
I'd also probably incorparate a thin detachable cover plate on front with slot in it using brush's like in other design just to protect the screw from debris, esp if main use is wood. For the little extra work and expense it will save the screw big time. You could use flexible consertina type covers but they are expensive and can be akward to setup.


EDIT: OR you could just cut a slot in the RSJ and mount the screw on back side and cover with thin plate, something like clear perspex so you can see workings.!! Added pic (RSJ4) with slot in couldnt be arsed to alter sides or move motors but again you'll get the idea.
It will weaken the gantry thou and some extra bracing along it's length would be required.