A client needs a light bulb changed over a stairwell so I need a way for a regular 4' stepladder to stand on a firm and level foundation. I'm building a rigid box 24" x 24" x 8" with one open side with gusset plates in the corners that doubles as a 24" high ladder base that goes two steps down from the upper support step. For differences in the steps I'll have plywood plates 1/4", 1/2" and 1" which can be placed under the box or the ladder and gives me a range of adjustment from 1/4" to 1-3/4" in 1/4" increments. I have a body harness from Grainger for tree work and so this with a rope fastened to the top of the staircase will be my backup in case of a problem. If it turns out to be too risky I'll still have a GP box.