[MrForbes]

Drill all the rivet holes in the gusset, then clamp the gusset to the bigger parts. Then drill the first hole all the way thru, and pop a rivet in it. Continue drilling and riveting one hole at a time, until there are two rivets in all the parts. then you can drill all the rest of the holes, and put in the rest of the rivets.

We've been doing this for building our "structures" for years....it's time consuming, but cheap and easy and doesn't take much equipment.