If you spend enough time building stuff you will undoubtedly come across the situation where you cannot get underneath something to trace the opening for a filler piece. I used to put a piece of paper and use back and forth pencil strokes to get the outline but it’s pretty inaccurate. I got the idea to use a clear piece of plastic and accurately mark the opening by taping it to adjoining pieces when I spied my package tape roll. Hmmm 🤔. It worked great, self explanatory pictures below.
Use a straight edge for a thin marking pen.
Apply the tape to the work to be cut.
Use a sharp Zona type saw on the line.
A little final sanding and you’re done.
Peter
