This is what’s used to roast green chile, but could easily be adapted to roasting coffee beans https://www.amazon.com/Chile-Roaster-Propane-Regulator-CR-BARBACOA-1/dp/B01M02H36S. You need a propane tank to accompany it.
Our kids got my husband a very small model electric coffee roaster for Christmas a few years ago, but it doesn’t get a dark enough roast– we like some of the oil coming to surface of the beans so that they have a dull, shiny look.
I like A.S.’s idea about using the grill!