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).
Green beans can be steamed or cooked in the microwave, mine has a setting for frozen veggies that really works pretty well. Or they can be just cooked for 15 - 20 minutes in salted water just to cover.