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).
Bring a large pot of water to boiling. Allow 1 gallon of water per pound of green beans. Fill a large bowl with ice water. Working in batches, carefully lower the green beans into the boiling water. Boil small beans for 2 minutes, medium beans for 3 minutes, and large beans for 4 minutes.