Jim:
I have tried corner clamps, specialty clamps advertised in the model train train magazines but always went back to the simple 12 inch bar clamp.
Purchase a small square (Micro Mark sells a thin version idea for this stuff), a cutting mat that has lines on it and a bunch of 12 inch clamps. Use the square and the lines on the cutting mat to keep things square while you glue. Use a little super glue on the top and bottom to hold the walls together, lightly clamp (don't over power this), check square then apply your plastic cement.
Joe
