I made this as a prototype before my friend made it out of steel on his CNC, but it proved to be very strong, even as a plastic part. 

Our bar diameter is 27 mm (a teeny bit more than an inch), but the holder has some tolerance, as expected. If printed with enough perimeters and plenty of infill, it can easily hold a kid or a low to mid-weight adult - I tested it myself (100 kg) a few times, and nothing bad happened :) 

Of course, you need to mirror the model to get both pieces. 

I'd recommend drilling a hole through the holder AND a bar once installed, so a security pin/screw can be installed; otherwise, the bar rotates and is very hard to control the grip.

I uploaded both .3mf and f3d, so you can modify if needed - apologies for an amateur model in terms of CAD practices, I am still learning :)

Have a great day!