Aligning extrusions seems to be the hardest part of building a frame kit. This jig simplifies getting cube corners right.
Step 1 - slide the "through" bar down the longer ribs
Step 2 - drop the other two sides onto the jig.
Step 3 - bolt the two sides into the upright (make sure the bars fit tightly against each other when you tighten them up)
Step 4 - remove the jig. It will slide off quite easily
Step 5 - bolt the remaining corner (make sure it fits tightly).

No fussing checking that faces are flush and that the bars haven't twisted.

I've also been looking at a model for printed corner brackets that I hope to share soon.