What is Reddit's opinion of KOWOOD 1/4" inch Shank 1-3/8" Height Multi-Slot milling Cutter bit (7 Bearings, Multiple Depths 1/8", 3/16",1/4", 5/16", 3/8", 7/16", 1/2") Interchangeable and Adjustable Bearings.?

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If the wood is like a picture frame, a rabbet router bit will have a set of bearings to choose from, to let you make a groove something like 1/8-1/2in from the edge, whatever depth you set it to (up to ~1/2 in below set).

If the wood is solid, my router came with a fence sort of attachment, which keeps the bit a set distance from an edge. You would set it from the far side, so any deviation would move you in to the waste, rather than the keep side.

Other choice could be a piece of plexiglass or thin ply, and a router inlay kit (harbor freight kit is fine for the brass parts, plan on ordering bits in bulk). You would need to make the template the first time, whether with a saw and a drill, or a razor blade, or a laser (I like the laser). You would then be able to make your own "tiles" out of a contrasting piece of wood.