You can use either 30" or 36" wide by 24 5/8" deep refrigerator cabinets, and use either the standard 2 doors, or you could even substitute drawers--just check the heights of the deep/shallow drawers to determine the best configuration for you.
From an engineering standpoint, you'll want to ensure there's sufficient support for the cabinets to hold the combined weight of multiple humans as well as whatever you have stored inside the cabinetry. The 24" deep refrigerator cabinets are only 15 1/4" high which conveniently allows for additional support underneath the cabinets in the form of a plinth box constructed of plywood. (Keep in mind that a comfortable height for a banquette is between 18" - 20", AND that any cushions that you use will add to the final height of the seat.)
Windowseat or bench: constructing using IKEA cabinets
Windowseat or bench: constructing using IKEA cabinets
Windowseat or bench: constructing using IKEA cabinets
