Big stuff for big kitties

First off, I have ragdolls, not Siberians. Feel free to ignore me if you must.

Everyone else, I need help! We just adopted a pair of 4 year old sisters. Vivi is a large cat but her sister Lulu is a medium sized dog. They both have a strong climbing instinct and, being ragdolls, can't really get a lot of outside time safely. So I'm hoping the owners of some other extremely large cat breeds might have some advice on stuff for big kitties.

Specifically I'm looking for sturdy cat trees/condos, especially if they have a cubby big enough for Lu. My guess is she needs at least 18"x18" of floor space inside the box and nothing I'm finding online is even close. Also, big covered beds just don't seem to exist. Lu probably needs a small dog bed, but those never seem to have covers and she loves to snuggle in a cave.

Any advice would be so appreciated.