Had another idea.

Rather than find something for the bottom of the ladder to sit on, which is going to be difficult, maybe bend the ladder 90 degrees and tie it off on top.

The base of the ladder will rest against the wall, the top of ladder will be roped to a sturdy tree on the other side of the shed.

I can even nail a 2x4 to the shed wall for the legs to rest on.

They make ladders with joints, my brother has one.