There are baby gates with clamp on blocks for bannisters.

I think you could clean up that gate clamp, smooth the rough edges, paint, replace the fasteners with exact size.

Try googling, 3.5” roll bar clamp. You may find a clamp designed for 3.5 tubes, with a surface for mounting. Designed for off road truck use, clamp attaches to bar, light attaches to clamp. You would attach the hinge of the gate to the clamp.

Some sort of hose clamp

- like for automotive use?

https://www.oneida-air.com/inventoryd.asp?item_no=ACB200000&_vsrefdom=adwords&gclid=EAIaIQobChMIrPSPy-uB1wIVy1mGCh1IWwjvEAQYASABEgLcvPD_BwE

Thanks for the tip, but the hose clamp doesn't really have a way to securely attach wood to it. I ended up buying a 3.5" D-ring that has a smooth curved end and screw-on nuts on the the two legs of the ring. I haven't hooked up the gate to it yet, but I'm hoping a piece of wood across the legs of the D-ring will be a secure anchor to screw the baby gate onto.

Thanks again!

The roll bar clamp was a great idea. I did not know about those. That also looks like it would work if my other plan doesn't come through. Thanks!!!