Building boxes with painted 3/8" plywood

[breckrider]

A friend has asked me to take 12"x14" painted pieces of plywood and make boxes out of them. What's the best way to miter the corners so the unpainted edge doesn't show then the best way to join them? Glue and pin nails? Just glue?

 

[Steve123]

Reinforcements along the corner. Full length. Glue and either nails or screws. Sand away paint on areas to be glued.
Angled reinforcements look better, but are a little more work.

 

[breckrider]

Reinforcements along the corner. Full length. Glue and either nails or screws. Sand away paint on areas to be glued.
Angled reinforcements look better, but are a little more work.

Thank you!