I couldn't find a 3d printable tool to expose the fretboard without taking off strings, so I made my own. The regular version should work with most 6 string guitars. If your guitar has a really wide neck, use the "long" version or scale up the normal version.

Print with 100% infill. You shouldn't have much string tension when using it, but it never hurts to have it be extra strong. I suggest putting a little felt or some kind of padding on the side that touches the neck to prevent any scratching if you slide it around.

Loosen your High and low E strings, put this around the back of your guitar neck, then put the low E into the hook on one side, then the high E into the hook on the other side. Then loosen the other 4 strings and pull them into the hooks. This should open up most of your fretboard for work and you can slide it around if you need full access to different areas.