We wanted to build a queen guest bed and roughly followed the japanese bed joint we used for our own bed. The room this bed was for was more compact and so we incorporated a headboard into the bed itself rather than having a separate structure.
A big puzzle was how to build in a middle support since we wanted the whole bed to be easily deconstructable (preventing us from using mortise and tenons or bolts to connect the center support into the headboard and footboard). Despite using oversized 1 1/8” slats we still clearly needed a centerboard for support. Without one the slates bowed considerably and could fairly easily be broken. We ended up building a freestanding center support with two feet. This completely supported the slats so I could walk on it with ease and allowed us to use a plenty wide support. All of the weight on it goes directly into the floor.
We screwed on a support railing for the slats to rest on, and we put in 3/8” pegs to line up the slats. On the underside of the slats we routed 1/2” slots to lock over the pegs.
Below are some plans and pictures:
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