Since making a demo version will require some time, time that could be used better in developing the final version of the game, there is an alternative.
On steam I’ve seen an Unreal Tournament game made freely available for a few days, once that time period expires the game becames unavailable you can only delete it.
So, no need to code, package and test a demo version. It’s the same game, freely available for a time period.
How does this sound?