How do I lock the exposure if I’m using back button focus? (Nikon D7200)

The AE-L/AF-L button on my Nikon D7200 is set to focus the camera. I like this setting, but then I don’t have a quick way to lock the exposure settings. The only way I know how to do it is to switch the camera to manual mode, turn off Auto ISO, then adjust the ISO to get the exposure I want. Is there a quicker way to lock the exposure and still use back button focusing? I also don’t want to use half-pressing the shutter button to lock the exposure. Can I set that “L” on the back that locks to focusing point to lock the exposure instead?