That is a simple box joint — but the example shown was cut w/ a laser w/ essentially zero kerf.
There are a couple of approaches:
- leave the corners rounded/uncut and use clamps to force things together — if you cut the corners with a very small endmill this can work well
- file the corners square by hand after cutting
- cut dogbones — this leaves visible voids if done w/ a large endmill — if one uses a very small endmill and cuts a not-quite dogbone this can verge on unnoticeable
- EDIT: use a V endmill or a roundover tool to relieve the edges of the joinery which would otherwise interfere: Using unsupported tooling in Carbide Create: Roundover / Cove / Radius bits
- use a vertical fixture and cut the joinery into the edges
- use a narrow V endmill and cut a hidden joint with voids
Do you explicitly want the joinery to show? Are you willing to build/source a vertical fixture? Would you be willing to source a narrow (1/8") 90 degree V endmill?