Setka A Cagarro

 The 2 min boat tool

If you dont know about this boat building aid here it is 

just use a flat  piece of wood jointed square, and cut the hole on a bandsaw,
a jigsaw works but make sure the cut is square and true

and use it like this

and if you need a smaller one just cut a smaller hole

use it for cutting the ply sheeting and it also helps for marking to plane the progressive bevels on the stringers