Notice how there is no full-height cover panel on the left side of the 24" deep x 30" wide fridge cabinet? We didn't want one, not wanting to 'box in' the fridge and impede ventilation. So we located the floor joists for the 2nd floor in the ceiling above the kitchen, and FIRMLY attached (one. two?) 2x6 to the joists above on the flat side for more screwing room, and to the cabinet below (using washers to increase the holding power). I don't remember if we had to add any spacers beyond that to close the gap. The cabinet was full and in the 8 years we lived there after the kitchen reno, it never move or even jiggled a little.