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while finish sanding, grain filling and sanding, and while applying shellac, say for french polishing? Thanks for the help. I haven't figured it out.

I use a few large thick bath towels, which I fold nicely and place on tabletop, or lay on my lap. Then I just hold the guitar with my spare hand. I rarely use a lot of lateral force doing things, so this is adequate. I never do any power sanding, I am too paranoid about over sanding something. I shake the towels out frequently and wash them when needed.

Many things you can do.
Often I just have the guitar body resting on a bath towel or the piece of carpet I use for this purpose which also protects the guitar.
You'll find that this may be enough to keep it from sliding around.

You can also build a cradle for the body which is also very helpful when routing binding channels. And
You can make a "vise" to hold the body when working on its sides.
Let's see if I can find pics of my setups.

Hey Oliver. I do all of the above! I often just do the towel on the work bench method, particularly if it’s cold or rainy. Otherwise I prefer to clamp a piece of plywood to my sawhorses, move everything outside, and let the wind keep the saw dust out of the shop.
The bottom photo above shows a large, vertical clamp for working on the body. They are easy to build, and incredibly useful for sanding sides, scraping bindings, and working on either end of the body. This is particularly helpful when routing the end strip channel. My shop is small, so, when not in use, I just take the outer clamp platen off, unscrew the pipe clamps and throw them all in a drawer.
Wishing you good luck and success!

Thanks for all the good ideas!