After laying out the frame, I drilled the holes.  I then inserted a 1/8"x1/4" pop rivet through the 2 pieces and clamped them together with a pair of Vise Grips.  I then "popped" them together.

Another picture of the riveting process.

After riveting the frame pieces together, I then peened them flat so they wouldn't tear the upholstery covering.