Option-up/option-down?

Two keys rather than one, but makes up for it by not being way off in some oddball part of the keyboard. You can one-hand it pretty easily, since there's an "option" right next to the arrow keys.

As a stolid classic-era Thinkpad user I don't have a dog in this fight, but it seems to me that the strain of having to hold down another key as I scroll rapidly would get tiring rather quickly. Perhaps if there was a Cmd lock it would be fine.