First I drew the dimensions out and made a jig.

Next I placed some tubes in the jig to simulate the longerons and crossmember. I then cut and welded the two angles shown.

Next I fit the two angles to the longeron and crossmember.

Next I fit the long tube that goes from the angle to the center of the crossmember.

Next I finish welded the juction of the long tube to the angle.	Here is a close-up of the welded junction.
Here's a picture of how it fits to the longeron.	The final product tacked in place.