Start with two stock 15-inch-high by 12-inch-deep double-door cabinets designed for use above a fridge (the width depends on the size of your nook). Screw cabinets together through their sides and set on 2x4 framing to support and raise them to seat height. Use 1x4 baseboards to hide the framing, and top cabinets with ¾-inch plywood trimmed with edge molding to create the seat. For a window seat that turns a corner (shown at left), add single- or double-door cabinets on the ends, supporting them from below and finishing the tops the same way as the center cabinets. Use filler strips to bridge gaps in front, and extra doors as end panels.
Note: Most stock wall cabinets are 12 inches deep, less than the suggested 16 to 20 inches for comfort. Make up the difference by furring out and supporting the cabinets from behind with doubled-up 24 cleats secured to wall studs. Use a wider plywood top that will rest on the cleats and overhang the cabinets by an inch or so in front. Fasten the cabinets and the top to the cleats, as well as to each other.