Quoting Tage Frid regarding what you are trying to do.
"I would never use a fully-housed dado joint. There are no shoulders to
lock the wood and help resist sideway stresses. Also, if the wood is sanded
after the joint is cut, the piece becomes too loose. If there are
imperfections in the wood, the piece will not fit tightly."
He suggests either tongue and groove, mortise and tenons, or a sliding
dovetail, but also notes that you must use a spline to use T&G in