Make elemental invokers stackable
Therefore I suggest making elemental invokers, sacks of spiders, and boxes of beasts stack. You could represent which ones are inert like this:
Alternatively you could have inert ones use a different stack, but then using an invoker with all 52 slots occupied would have to make it fall out of your inventory, so I prefer the former option.
I am sure someone will object to doing this for sacks of spiders and boxes of beasts because you'd need to manually keep track of how many charges have been used...but currently this is already the case, as unlike wands they aren't inscribed with the number of uses. Since blood is allowed to stack, I don't see an issue here.