New Boy introduction

[mekanik]

Hi Paul
Others (who have actually built a machine) would be better to give advice regarding gantry weight ect, but as you say you want it as rigid as possible and also not too heavy.As a general rule the axis are usually X longest Y gantry and Z for the virtical. our cousins from across the pond sometimes do it differently, it,s a pitty you didn't open a discusion before you started your build as if you are strugling with floor space you had the option of making a semi vertical layout.
Good luck on whatever you decide, under no circumstances purchase any of your electrical gear before you have your design set, the weight of the gantry is an important factor regarding motor choice, and it would be better for you offer your design up for critique, the lads will put you right regarding motor selection.
As you want to use the box section you have already purchased for the gantry i would do as you said and put a length of flat bar inside across the narrow section, drill a series of holes in the box section and stitch it to the flatbar so get the same effact as two square sections, won't be as rigid as the latter but should be better than the box section as it is now.
Regards
Mike