I designed this portable flat chess set to make it portable and easy to carry around.

There is no clearance for the stopper to be inserted in the lid. It is a tight fit because I wanted to ensure that the stopper remains in place over time. If you would like a more loose fit I would recommend a 0.5mm to 0.8mm clearance.

Also, I was worried that the slotted opening in the lid for the stopper would be too close to the board so that it would not close, so I added 1mm or 2mm to leave a gap, but it was not needed. You could make it a tighter fit.