I've also been away for five weeks. Love your XR rig - that's another level up from mine. Great work!
In terms of the cross members under the frame I found that 1-1/4" square tube makes for a slight press fit into the cross members' U-shape and stiffens them. I tested the rig thoroughly at various speeds and with different loads, and when I took the hack off the other day (I put it up for sale but that's another story) there were no signs of deformation or damage of the cross members. The reason for my approach was that I did not want to weld on my frame.
What are the tires you have on your XR - I like them.