Hi John
For something quick and simple, and ought to be available pretty cheaply as well, what about a roller skate or skateboard wheel. They normally have ball races for bearings and can stand quite a large weight acting on them as well as shock loads so should be easily capable of tensioning a belt.
It might be worth talking to your local garden machine supplier as well, the larger rotary mowers, esp. ride-on size, often have belt tensioning pulleys fitted under the deck as part of the transmission. I’ve used one of these on the drive to my lathe BUT it fits inside the belt not outside as you require. I’m sure outside jockies are available though, have a look on E-bay.
Keith