You have a few options available If you are running a tensioner go with the melvin and leave the front dérailleur on and two chainrings. or a regular tensioner, And leave to cogs on the back separate them with a spacer And you will have to get off and adjust the tensioner to switch cogs, with both options you will only have a few teeth's difference.

Lastly if you are not running a tensioner check out the white industries DOS eno freewheel

Or lastly man up and mash it