Wood will expand and contract across the grain, and a wide panel made of solid wood could change width by a half of an inch, warping the door frame. By allowing the wood panel to float, it can expand and contract without damaging the door.
Put trimmed and cleaned green beans in the steamer basket, set over the boiling water, cover, and steam until tender to the bite, 3 to 5 minutes (depending, again, on how crisp or tender you like your green beans).
Use the paring knife to cut the ends off of the string beans. Cut the beans in half or 1-inch slices, if desired. Bring the salt and water to a boil in the saucepan over high heat. With a vegetable steamer: place the steamer above the boiling water so that the water does not enter the steamer. Fill the steamer with the prepared string beans.