Well here goes... Aluminium frame router design

[IanS1]

This is what I had in mind, There will be aluminium blocks in each end of the X box section tapped to accept the the two bolts through the legs?

As Jazz as pointed out, the more adjustability you have, the better. Cutting and fixing the ends of the X box section the way you have drawn could make the adjustability more restrictive. Also, bolting through the end uprights may cause the frame to warp as you tighten the bolts.