Removable side of a mitered frame

I'm going to be making a framed panel for doing jigsaw puzzles. Basic design is a mitered frame with a dado for a sheet of hardboard. The puzzle sits on the hardboard, and is retained by the frame.

I would like to be able to remove the frame piece from one end, so that the puzzle can be slid out as a unit.

Rigidity is important - but only to a certain extent. It's just a temporary worm surface.

A few options I thought of are:

1. Leave one side unglued so it can be removed and reattached as needed. No fasteners at all.

2. Leave the side unglued, and attach a hinge to the outside of one of the corners so that it can swing out.

3. Cut a through slot thick enough for the puzzle to slide *through* the frame piece.

Thoughts?