Something like this:

A horrible mock-up of what it could hopefully look better than in-game:

Essentially a small stone trim that runs along the edge of a tile and and is built like other fences, except it doesn't disallow passing through; it just looks nice. If it snaps to edges of tiles rather than centres, it could potentially also be made so that it can be built in the same tiles as other walls, so you can add an edging or trim to your larger walls as well.
Brick trimming could be added in a similar manner if you want to go all out. I think it'd add to a lot to the aesthetic of bases with something pretty simple.
Please.