I fabricated custom length tools a few times for cleating top cracks in mandolins and archtops telling myself it was ok since I didn't do this type of repair often. Truth is though I was doing this kind of repair often enough that I needed a general purpose, reusable tool.
To use this tool, tape the outside magnet holder with the "wings" on the top over the crack. I put a piece of double stick tape over the inside magnet holder to hold the cleat. The double stick tape should be long enough to wrap over the sides of the inside magnet holder so it stays stuck to the holder and not the cleat when you remove the tool. After a nice cleat installation you don't want a piece of double stick tape stuck to the bottom of the cleat that you can't reach!
The non-printed parts needed are two neodymium magnets 1/2" or 1" long, and a 4-40 x 1/4" screw to hold the telescoping arm from sliding out of the inside magnet holder.
I printed mine with PLA. Make sure you enable "support everywhere" in your slicer for the slot in the telescoping handle.