Edge drilling jig
This is a very simple jig that makes either marking or drilling holes into the exact centre of a panel very easy.
The design is very simple, and all you need is a piece of wood with smooth parallel sides, and some dowels to make one.
A total length of around 4" cut from some 1 1/2" square stock would be fine. Carefully mark a centre hole in the middle of one side, and two further holes equal distances from the centre hole. These need to be drilled on a pillar drill to ensure they are perfectly square. Once the holes are drilled, glue some lengths of straight round dowel into the two outer holes:
To use the jig, place it on the edge of a board with one dowel either side. Rotate it until both dowels are flat against their side of the panel. The centre hole will now be dead centre on the panel edge, and can be used to guide a drill. Alternatively fit the centre hole with a marking spike of pencil and use it to scribe a centre line: