Here is a small box that can be used to protect a single DnD miniature of about 28mm scale.  It comes in four variations.  One is a simple box, and the other three have a slot in the bottom to hold the base of the figure, each for a different thickness of base. The lid snaps on.  Two versions of the lid, with and without my store name on the inside.  The blender source files are available if you want to modify it.

The basic size is 2" high by 1.5" by 1.5".  It can be scaled in the slicer to fit most miniatures as there is nothing in this that is too scale dependent.  Scale the lid the same amount as the box and it should still hold tightly.  If the scale is too much, shrink or enlarge the lid a couple percent more to restore tightness.

I use these for shipping painted miniatures securely for my Etsy store (https://ABitMoreAdventure.etsy.com).  I can custom print each one to fit the miniature.  I add some padding inside, and a rubber band around the outside to more securely hold the lid in place.  Then it can ship in a padded envelope or small box without risk of being crushed by the post office.

I print with PLA+, no supports, any infill, .20 layer thickness.  Just about any settings would work for this very simple design.