Quick update: I got the "cramp buster" off my other bike that I used years ago. It's basically a plastic part that wraps around the grip and allows you to simply rest your wrist on it to hold the throttle.
It didn't fit tight enough, so I took a strip of metal and forced it under the grip to make it fatter at the end and it works.
I'm able to stop holding the throttle with my fingers and just rest some weight on the plastic paddle. It's like the others, but uses your wrist instead of the brake lever, which allows you to adjust on the fly.
The down side is getting the correct tension against the grip, I might add a screw to it to make it tighter, but it does help with long runs.