I couldn't find an insert that accomplished what I wanted so I took on the design myself. 

Here is my first insert designed and printed entirely by me!

This insert holds 12 characters (but no map) in a Marvel Unmatched box with no lid lift. The same insert can be used in a standard four character box with room for three maps with no lid lift. This allows one to consolidate three standard four character boxes to one box if so desired!

I used 24 6x2mm magnets but only 12 are required for the insert to function properly.