I thought about something that could be DIY'ed but you would need benchmark specs.
Three ¾" rollerised tubes mounted square onto a steel plate, with two riding on the belt's flat side and the third between them on the cogged face. This single roller would be on a cam adjustment, like a timing belt tensioner bearing. To get the belt tension you would use your inch-pounder on the cam-mounted roller tube. When the depressed portion of the belt just touched an anvil on the flat side and clicked off the inch-pounder, you'd be close enough.