Cut a piece of plywood or MDF the depth of your file cabinet by the width you want for your desk. Attach one end of the board to the wall and lay the other on the file cabinet or put the file cabinet against the wall and cut a top and leg from the board. The "leg" would be the same width as the top and the exact height as the file cabinet.
What other Hometalkers did.
Buy a hollow core door to fit on top of the filing cabinets. Place cabinets at each end of the door leaving knee space between. No need for cutting or screwing things together.