A thread in the Lands of Lore section points out that Boxer does not have the built-in key mapper function that DOSBox provides in Windows. Here's a possible solution for that. It Works For Me[TM].

Look up Karabiner, which is a general-purpose key mapper for macOS.

I've used it successfully to map WASD to arrow keys in Magic Carpet. Yay, i can reach the number keys now!

Karabiner allows you to configure multiple profiles, so you can make them per-game. Also check out the checkbox to flip the function key Fn modifier so you don't have to press a chord to strike an F key! Good, right?