Well here goes... Aluminium frame router design

[toomast]

I would not use steel flat-bar in there. I understand that it is easier to build it like that but I doubt you gain much with this design. In order for diagonals to work effectively other ends must be connected as well. So I would advise box section for diagonals + connect the other ends with box section too. For diagonals you can use smaller profile. If you want to keep adjustment option then you must weld the end-plates and use bolts.
I'm currently redesigning my machines frame - adding connections for the other end of the diagonals and making profile smaller but thicker.

[Boyan Silyavski]

I would use no diagonals at all as the structure seems right to me. However i will use 10mm thick plate for the bolt on connections. That should do it