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The idea is to make a simple I-beam shaped piece slightly wider than the cut stone walls. It allows simple stacking of two walls on top of one another without requiring openlock slots or special pegs on the top of the walls (which would get annoying any time you're not stacking walls.) Quite simply, you lay out the first floor of your building, then add these I beams at the corners and sides and stack the next layer of walls above. It's not perfectly sturdy, but should give plenty of stability for most buildings, especially since a building of up to 8x8 tiles can be accommodated with just 4 corner pieces.

The texture of the piece could be either 2 layers of cut stone (for simplicity) in which case, sticklers for detail could flip their pieces upside down to make sure their grout lines lined up, or, since there would be a .1" bump out, you could just roll with it and make it more of an architectural feature. Imagine the supporting structure you would see under a ledge, or the structure that held up overhanging crenelated edges on a medieval castle. I wish I knew the architectural term or could draw better.

You could probably get away with the entire thing in 9 files (until someone liked it and wanted curved walls too.) It could even be the basis of a roof system.