Umbrella
As a hat, four people holding a cross use an umbrella. The umbrella is big enough to be a hat for all four people, so it makes sense. All four of them are religious missionaries, so that’s why they’re holding the cross.
See these four people all under one umbrella. No one is carrying the umbrella, it’s just resting on all of their heads like a hat. Their hands are busy holding that cross, anyway.
さん
かさ
None
You know the Spanish phrase “Mi casa (かさ) es su casa”? Well this is kind of like that, but only with a hat. And that hat is an umbrella. Mi casa es yours too, because it’s an umbrella and also a hat. This makes sense, trust me.
Imagine the four people in the umbrella hat again, only this time imagine them skipping along singing “Mi casa es su casa”. It’ll help you remember.