Bed Slats

I am currently designing a Bunk bed and would like to know opinions of what wood to use for slats and how to keep the side rails from bowing over time.

I have decided on 1.5” x 6” side rails on top and bottom bunks with dowels set into a cleat and slots at the end of each slat to prevent them moving. I have seen some ideas of dovetailing the slats into the side rail or an attached ledger, but wondered if anyone else had any ideas on how to stop the rails from bowing?

The beds will be made of Poplar and painted. Should I use poplar for the slats also? If not, what would you suggest (that won’t cost an arm and a leg)? And how thick/wide should I make them?

Thanks in advance for the help!

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