Similar topics have been posted before. But I am proposing a slightly different solution.
Instead of allowing users to customize all channels in Guest Mode, allow a limited number of custom channels to be included. (perhaps up to 5?) They would need to show up at the top and not lost in the mass of default channels. That could be in the shortcuts area or in a special section at the top.
I realize that default channels in Guest Mode is likely a source of potential revenue for Roku. So this solution benefits both Roku and the user who wants to provide the service to their guests.
In my use example, I provide access to live, local OTA stations via an Emby server. Asking my guests to install the Emby channel each time is not practical. So I cannot currently use Guest Mode.