What I do is just lay my hand in my thigh and if that doesn’t work just try two hands on the reins until you get used to it
Maybe tuck it under your leg, like behind your thigh so it's sandwiched between your leg and the saddle?
Try to keep your elbow of the free arm glued to your side. Pretend that it actually is glued to you. It helps a lot.
When I ride western on occasion, I either hold my free arm up as if I was using both, or I rest it on my thigh. If I don't feel it on my thigh I know I'm probably flopping it around so I lay it back down. Hope this helps!
I have the same problem when I have my riding lesson and when I ride one handed I hold my free hand as if I was riding with two hands