I've seen something similar to this commercially available, but shouldn't be too hard to home-brew:
1. Take two pieces of right-angle material (steel, aluminum, brass?), both about 2" longer than what they'll be clamping.
2. Except for 1" at each end, remove enough material at the angle to allow access for applying glue, etc.
3. Put the two walls to be glued between the angles and use clamps of some kind to hold the angles together.
4. Apply the glue through the access openings described in Step 2.
Here's a (very) rough illustration. The red and yellow part are the two angles, the green is one of the walls.