These grid tiles are about 30% smaller than others on Thingiverse, but still fully support 9 Carcassonne tiles. The reduced size frees up space in the box for the other pieces.
At 2mm thick, you can comfortably stack 30 in the box, which can support at least 143 tiles (almost double what comes in the box).
The tiles are symmetric, so as long they're right-side up, they'll fit together.
They're purposely flimsy, to accommodate printing errors, and since they use so little material (3g), you won't mind rebuilding any that break.

I've also made a Carcassone Grid Tile Tree to hold the tiles while not in use, and a Carcassonne Box Organizer Insert to organize everything.
It's all together here.