Double and Single Frames, pillows, and/or curtains can be found in Comfort > Misc.
Simple Metal Frame Double
Simple Metal Frame Single
I made these so I could create some sofa beds. I found that my separated mattresses will slip over any loveseat or living chair using the MOO cheat. The sims can still route to and use the beds properly while the footprint of the bed keeps sims from sitting on the seats and creating an awkward situation for anyone trying to sleep.
These also work well on their own as a super cheap bed frame. Good for those sims who can’t afford much.