This woodworking jig is made from MDF. Wooden edge strips are limiting the movement of the round router base plate so that an 8 mm bit is creating a 15 mm wide mortise.
The length of the mortise can be adjusted by moving the stop block in its groove. Another moving part is the plywood support at the bottom which enables making mortises at a constant distance from the edge of the work piece.
The opening on the MDF board is done with the router. Positioning this jig is very easy as the mortises are routed exactly along the edges of the this groove.
I also marked a scale for mortise length next to the hole and the stop block.