When I want to inspect my 300TD tank screen, I prepare a plug using a golf ball wrapped in a rag in a plastic bag to jam up against the hole after removing the screen as quickly as I can. I prop it up with blocks of wood or a jack. I only lose a cup or so of fuel, which falls into a wide pan underneath.
I've done this a couple of times; the screen has been almost perfectly clean. The amount of fuel in the tank is not an issue for me. Steve