The vertical rail is a nato rail. I can attach this handle to it: https://www.amazon.com/SMALLRIG-Camera-Anti-Off-Designed-Spanner/dp/B07X1QKB1Z/

The handle is great since it spins and can be a top handle or side handle: https://m.media-amazon.com/images/I/7192GmVmQlL._AC_SL1500_.jpg

But it's totally overpriced, I got it for 20% off. Prime Day is coming up, wait until then if you're not in a hurry.

And I attached a short nato rail to the top of the cage so that I can move it there quickly. I like that you can easily spin the handle forward if you have a heavy lens.