Those of you with custom sleeping platforms in your bed? How do you keep your lower level mattress dry?

I’m thinking of buying BamBeds and a mattress to sleep in the lower level since my family is growing. My concern is water and dust intrusion. I’ve never had to worry about it before. How do you guys keep your custom sleeping platforms dry?

I don’t have any water intrusion that is that bad that my mattress downstairs doesn’t get wet. I barely get any TBH, only when washing and the odd heavy downpour while driving.