Essentially you make entity that has its own inventory. You can add supply zone parameters for it so that there is a limit on ammo it can supply.

I've made more advanced version of ammo box in this mod: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1966228928

There are three kinds: small arms, grenades (hand and grenade launcher ammo) and explosives (c4, mine).

On top of that some effects are attached on destruction of such boxes, so they explode. Explosives supply box is more potent, so can be used instead of a normal mine (it is about 10x stronger than normal claymore). So a truck can ride over it and it will explode. However, armored units will get their tracks damaged and some armor, depending on vehicle.

So in essence a soldier can equip ammo box and walk around resupplying units. Or same ammo box can be dropped on ground, however cannot be picked up. Infantry can access inventory of a box to see goodies inside. And there is always that special use to make it a mine if other vehicles ride over it.