Contest: Design a Jig

Here is a challenge for you Techneakians!

I need to weld my 2 main tubes for the Ruzter Trike! They have to be aligned square on about 5 different planes!

The contest is to suggest the easiest way to build a jig that ensures the tubes are square to each other and the ground! Assume you have a flat surface to work on.

Here are the 2 pieces and some pics of how they are supposed to fit together (they are not square in the pics). One note that might not be obvious. the cross tube (smaller V shaped tube) has 2 flat plates welded on the ends. Two of the edges of that plate will be perpendicular to the floor when properly aligned (look at the second pic), but the other 2 edges (top and bottom) are cut irregularly and can be used for aligning. The bolt holes in the plates are drilled exactly the same and are at 12 degrees to perpendicular.

Post your ideas in the comments, of if you have a drawing, post it as a separate post!

Thanks, questions welcome.