What is that rule for?
I might be a weird man, but I love reading tabletop rpgs rules and mechanics. I love to see how different people tackle the same problems in different ways. Ironically, maybe, my preference is for rules-lite games, so a good rule to me must create a sense of coherence in the game world and be easily applicable during gameplay.
I'll take inventory management as an example, since I recently wrote a Backpack!, a one-page system to do so. Inventory management in old-school game, to me, is mostly about solving the question of how much treasure can you bring back? (with a side of how many tools can you have with you?). Slot-based encumbrance is a great way to simplify it: it abstracts both weight and bulk in a very simple way and makes the tracking of things much easier because it keeps the item number lower.
That's what this rule is for: track what a character can reasonably haul on their person without too much accounting. It's important to keep it in mind, because at some point, the system stops to do so, and another system might be better.
In Backpack!, I specify that carts and bags of holding do not use the same slot-based system. Why? Because the number of slots becomes so high that it becomes a detailed spreadsheet again. Tracking 10 slots on a PC makes it manageable; tracking 200+ slots for your caravan does not. And whatever made sense on an individual level (big weapons taking 2 slots 50 coins filling a slot) does not necessarily do so in this different context.
Even more importantly, tracking what is that many slots is not useful: it's not really about the same question at all, even if we are still talking about items being hauled. The bag of holding basically erases the need to track treasure and tools for adventurers, and that can be fine. If you still want to do so, maybe a better question is "what types of things can be stored in it?" If only small items (like jewelry and coins) can fit in the opening, it's a very different tool than if you can fit a boulder.
For carts, the question might be about trade or building material; "slots" could be interesting if they track this scale instead of what an individual can haul on their back. If your game want to give a lot of importance to this aspect, devising another "cargo-tracking system" can be useful (like the one used for ships in Stars Without Numbers). But using individual inventory slots to track this is not a great idea.
Rules must work for you, not the other way around.