Cut two planks of 2-by-2 lumber to the depth of the kitchen cabinets you have chosen, and glue the lumber to the outer edges of the cabinet top. This creates a flat, sturdy surface for the door and serves as a screw anchor for the door.
Secure the lumber in place with four evenly-spaced 2 1/2-inch screws through the lumber into the cabinet.
Position the cabinets where you want the desk to go, about 40 inches apart. Place a flat, hollow-core interior door over the top of the cabinets, centering it.
Secure the door to the cabinets with four evenly-spaced screws through the top of the cabinets and 2-by-2 lumber, into the underside of the door. Stagger the screws so they don’t hit the other screws that you drilled into the wood.