Need some help with some ideas for something (cantrip?) buffs ranged damage type thing

So I have a campaign I'm working on setting up and one of the players wants to be a monk who uses a sling as one of their weapons. As is the sling is kind of weak and honestly not terribly realistic in a sense so we're looking at ways to buff this up. One idea I had was adding ammo as a limited use/turn type thing. If this was just on paper it'd be easy to track, however due to things we're making heavy use of DnD Beyond and the character sheets, vtt, etc etc. 

While I'm able to pretty easily figure out custom attacks/weapon style things what I'm really hoping here is like some way to give them a spell/cantrip/item that when used buffs/changes their ranged damage in some way (like maybe a "bigger stone" that deals 1d6 rather than the base 1d4) for a limited time to simulate the ammo. Unfortunately I'm not too familiar with the homebrew tools here and so I'm running into some issues with getting things to function like I want. Like I can make an item and give it to the character, but then I can't figure out how to "use" that item so that it buffs the damage of the base sling attack. Or I can make a feat that seemingly gives some things, but then it's global or not limited use type thing.

Ideally I'd like them to spend an action to swap ammo out also.

Any ideas or places I should look/start?