When I was leveling frets my guitar would slide around a lot which was scratching the back of the box. I put it in my bench vise to hold it steady, but the angle was awkward and if I wanted to turn the instrument around to face the opposite way, I couldn't.

After noodling on it awhile I came up with a tool to lay the guitar on that was padded so the box wouldn't get scratched and had a way to clamp the neck down so the guitar wouldn't move around.

There isn't a picture but I drilled a "dog" hole in my bench top and a corresponding hole in the tail of the clamp and use a dowel to be certain the clamp doesn't slide around.  

I threw this together in a few hours out of scrap wood and parts I had laying around.  I didn't have to buy a thing.  If you want to build this and buy everything it wouldn't cost more than $10 including a new bottle of glue.

I won't go into great detail because the pics are pretty self-explanatory. If anyone has questions please feel free to ask.

This photo shows the completed clamp with my latest uke on it:

This is the completed clamp empty:

Here is the detail of the adjustable headpiece.  I may rebuild this because I want to increase the length for longer necks and still support the guitar under the nut.  That will require the guitar to rest high enough in the neck cradle so that the headstock does not lie on the board, especially scarf jointed headstocks.

I made knobs out of scrap pieces of oak and threaded rods. You could use wing nuts but I ran out.  I cut the rods to length and glued them into the knobs.  I used an all purpose glue.  Wood won't work.  Gorilla glue or epoxy are good alternatives.  I used the pound in threaded sleeves to receive the threaded rod.