Hi Stephen,
What I did was not make the joint fit tight. I left just a small amount of play for the panels to fit in the corners, then I used polyurethane glue. I applied the glue in the corners and dampened the panels lightly with water and pushed them in place. Do the main panel first and the two smaller panels last. The glue will fill in any gaps to make a nice tight fit.

That's good to hear. You're getting close to having the speakers finished. Get pictures posted for everybody.