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?

The simplest option would be to make a copy of a Sling and adding the Versatile weapon property to it. You can add those from the "Modifiers" tab. You can add multiple properties to the weapon, including "ignore weapon properties", if you don't want it to use ammunition or similar.

How to handle the versatile ammo then would be stated in the description of the item and it would just be up to you and the player to keep track of which version of the damage roll you're using.

Is there a reason why you can't simply give them the magic stone cantrip? Alternatively, you could give them a feat that allows them to cast divine favor a number of times each day.

There is already a variety of magical ammo available: Ammunition of Slaying, Walloping Ammunition, +1/+2/+3 Ammunition. You could also simply allow them to use potions & similar items as ammo for their sling, such as Alchemist's Fire, Acid Flask, Holy Water, 

Quote from Agilemind >>

Is there a reason why you can't simply give them the magic stone cantrip? Alternatively, you could give them a feat that allows them to cast divine favor a number of times each day.

There is already a variety of magical ammo available: Ammunition of Slaying, Walloping Ammunition, +1/+2/+3 Ammunition. You could also simply allow them to use potions & similar items as ammo for their sling, such as Alchemist's Fire, Acid Flask, Holy Water, 

Did not know of that cantrip for one, however for two I've since learned that I think what I was looking for just isn't possible really. I was really hoping for something like making a custom sling that did like 2d4 and could be rolled from the character sheet automatically rather than adding another button, but I don't think it's possible with the way things are setup sadly. Same thing with the ammo, even if I "equip" a sling bullet +1 it doesn't seem to automatically add the to hit and damage to the roll, it's still just the base 1d4+3 whether the +1 ammo is equipped or not. Yes it's easy enough to just remember to add the +1, but at the same time it'd be nice if it was just factored into the base roller.  

That said I like the idea of letting them hurl items with their sling so may let them choose to do that. Probably sans damage roll for the sling itself even though I imagine it's not fun getting a glass vial hurled at you lol. Maybe 1d4 slashing for shards of glass? Meh.