Nail or screw a piece of wood along the bottom leg area… fill the holes afterwards with a bit of roof putty…
A couple options;
1- User ladder leveling lags. Good for extension ladders. I use them on all ladders ranging from lightweight 12ft – industrial 28 ft.
2- Pivit Ladder Leveler, this is a neet tool. I haven't used one yet, no need right now, but see them in the paint store all the time. Might be a good option.