We did this on
our roller claw.
Inside the square tubing, we drilled holes for 1/2" ID bronze bushings. (We used bronze bushings to ensure we had adequate material left on the tubing; ball bearings would've been too big to fit, and we had more than adequate power to make the lower efficiency a non-issue.) From there, we used #25 chain and 1:1 sprockets to go from the gearbox to powering the hex shaft. In a perfect world, you could add a keyway to the part of the shaft to add a key to match the sprocket, but we got away with grinding a flat spot down on the sprocket part of the shaft and using the sprockets' set screws to drive the rollers. (Mind you, we didn't use the claw as much as some teams. I can only encourage you to test on your own and draw your own conclusions on longevity.)
We used 1/2" hex stock, turned down on a lathe to fit inside the bushings, then made up some plastic inserts to fit inside PVC pipe and broached it to match the hex shaft. (Since we were riveting it all together, we did NOT want to go fishing for a lost key. Besides, we had a broach.) I want to say we added E-clips to these, but I can't confirm it from the photos. Even without them, the riveted claw structure should do a lot to constrain the roller on the shaft.
Our claw had its fair share of issues, but this aspect of it was fine.
Hope this helps!