I started modifying my electric guitars, so I looked into youtube tutorials and quickly found that a luthier has dedicated tools.
One of them is a neck rest, to reduce the pressure on the headstock when working on a guitar.
But I though, hey, you don't have to buy that...
Step 1: Get a Decent Base
I used a "mymuesli" roll. Any poster roll (the one you would use for sending a poster by Mail) would do as long as it's made from cardboard.
Cut it to approx. 15-20cm. height.
Step 2: Fold It Inwards
Fold the top inwards as shown in the picture.
Step 3: Upholstry
I used a (fresh) kitchen sponge, cut of about 1-2 cm of the top and glued it (lightly) on top so that the guitar neck rests gently.
Step 4: Use Duct Tape...
...to secure it and to make it look "smooth".
Use as much duct tape as you like. Make sure that you cover the complete top section to that you don't get grease or dust in there when working on guitar. Also be sure to not tighten the duct tape stripes to much or the upholstry effect of the sponge is lost.
Step 5: Done. Use It
Use it to do fret work, change strings, work on the tuning machines or whatever you need to do.