Cartographers insert (edited)

The original by [buckstah](https://www.thingiverse.com/buckstah) didn't fit my idea and the remix by [Mark2dot0](https://www.thingiverse.com/Mark2dot0) didn't fit my sleeved cards perfectly (there was some friction). Additionally it wasn't sturdy enough for my taste and was bending and getting stuck while getting out of the box.

I've shrank the thickness of the sides to fit better slightly larger sleeves and enlarged the thickness of the bottom. I've also connected the center with the two remaining sides for extra sturdiness.