I just measured the opening at the top of the stairs. The support was 1x3 pt that i had laying around . I put 3 short pieces across the top step and one on either side of the entry door (measured from the bottom of the door to just below the edge where it would make it flush when the plywood was resting on it). It is easy to remove and put in place. I mounted the 1x3 with velcro so i could.remove without doung any damage. Has worked out great.
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