This is going to sound strange. Find a junker guitar for as little as possible and unscrew the bolts or cut the neck off depending on what guitar you get. Glue a dowel to the cut end as a handle for the right hand. Now you have a neck with no body to practice on that you can take with you. You can cut it down to the 12th fret and cut off most of the headstock. You would end up with a neck that was about a foot and a half long.
There is a product on the market that is about 8 inches long made out of some kind of rubber. It looks terrible and only offers you a five or 6 fret span. About the only thing going for it is its' small size.
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