With the back edge of that digging into the cove, what also happens (even if you put a bath mat under) is that a lot more pressure is exerted on a smaller spot since the weight in not evenly distributed across the entire base of the ladder.
What I would do is get a piece of plastic or wood the same width as the ladder, but not as deep as the ladder (2 feet x 6 inches?? approx), mount it under the ladder towards the front, this should give you a bit of a space where the ladder is hitting the cove. It will raise the ladder and inch or so, but should still work fine - make sure you really round the edges good - this should give you a good surface area for distributing the weight.