Adding object slots to another object- not deco

Hi! I am trying to add some different slots to the treehouse that came with Growing Together. I want to be able to place things on it like end tables, bookshelves, plants, guitar, etc. I'm having a hard time finding what type of slots I need to use. I've watched a ton of videos and read through posts here but nothing seems to be quite right for what I need. Anyone know what slot type I'd need for this? Thanks!

I'm also super new to this so please bear with me, I'm still learning!