There are all sorts of things that you can bolt on to the ladder, but I like something else. I have an old Bell Telephone ladder leveling block/wedge. It's about 7x12 and is about six inches high at the highest point. There are steps on the top edge that allow the ladder to be leveled. It serves to level the ladder and distribute the load over a wider area keeping the ladder from sinking in.
The bolt on levelers, some of them, can compensate for a big difference in leg heights. The block doesn't do that, but it doesn't add to the ladder weight when you're not using it. It's a nice thing to have and very easy to make. It works well with any ladder.
R