I want to add a slot somewhere in the player's inventory that only my Item fits in. I can't quite get coolAlias' tutorial to work. Can anyone help?

Create a custom slot class that extends Slot. Override isItemValid(ItemStack stack) and return true if the itemstack contains your item.

 

I think there is more problem with:

"I want to add a slot somewhere in the player's inventory"

 

I'd like to hook into question author had in mind: How can one add slot to EXISTING container?

 

Now I am cancelling normal container opening and opening my own instead - which loads both: slots from normal one and my slots from ExtendedProps. Is there other way of doing this? (forge-like - compatible with other mods)

You can use PlayerOpenContainerEvent to do that. Sadly that event does not actually fire when the player is opening a container, but every tick, as long as a container is open (which is all the time, because when you are not looking at anything else, Minecraft assumes you are looking at your inventory, even if it's not actually open).

 

I recall that there's a way to determine if you're not actually using your inventory.....wait, no.  I was doing something else....never mind.