I made the tube frame first and then fit the rest inside. It is more or less centered on the tubing by raising it up on the glass working surface I use with some shims until it is about centered then touch the joint with ACC. It requires a good fit but with a little care and patience it works well. One thing I did do however was this;
I had to cut some out of the rear tubing and pull the ends back together in the middle. When I got it the way I wanted it I glued the ends back together. This would have left a real weak joint but that was solved by cutting a slot with my Dremel along the tubing on both sides of the joint and installing a busquet (sp?) like you would in wood working. One on top the other on the bottom. With the joint sanded smooth it was stiffened as if I had never cut it.