A doweling jig for 8mm dowels with 50mm spacing between holes, the clamps (print two) can be used to center it on or along the edge of a board.
If you remove the clamps you can use the slots on the sides to align the jig to dowels that are already in place.

It needs six 8x22x7 bearings (standard skateboard bearins) and at least four M4 screws and nuts for the clamps.