Summary

I made this corner piece for closing off boxes for display cases. These are 40mm in every direction and work with corrugated cardboard up to a few (5?) mm. Since cardboard can squish a bit, thickness is not crucial, as long as it's not double walled (that'd be too thick)
Friction from the cardboard is usually enough to hold the box closed, but a dab of Uhu or white glue will solve it if not.

A second file includes a smaller version, 25mm on the sides x 40mm tall.