Centre finder

Lots of these around, but this design does away with the need to hold three things with two hands. The pencil is loaded forward by two elastic bands leaving the user two hands to guide the tool along the length of the wood.
This is designed for hexagonal shaped pencils most of which should fit although you might have to lightly sandpaper the sides if it is too tight in the holder, no big deal though.
I have used standard metal dowels for the guide pins which can be bought easily and cheaply on Ebay.
Four sizes, but unless you are really keen to print the largest one, the three smaller one will suffice for most standard timber sizes.
Use thin light elastic bands(2)