My first instinct was to return these.  The trays do not attach to the base and the base frame doesn’t lock in place either, despite the little round gadget they sent to stabilize it which is useless.  We liked the height and the idea of them, so my husband drilled small holes in the top tray in order to zip tie the tray to the frame to stabilize it and allow it to be moved without dismantling the whole thing .  Holes will also allow drainage.  They’ll work for our purpose but could stand manufacturing structural improvement for sure.