Extension ladders have aluminum rung locks so you can raise and lower simply by pulling a rope, and safely locking in place when the right height is reached. When lowering the extension, a pair of spring-loaded plastic "flippers" push the rung locks out of the way.
I recently acquired an old 32' extension ladder with a broken flipper and rather than buy a replacement kit, designed a flipper in OpenScad and printed a pair on my Printrbot simple. It works very well.
Installing these can be tricky, but there are youtube videos like this: https://www.youtube.com/watch?v=-9dIPjSjOlw .
I printed mine at 20% density with an extra-wide brim for adhesion since I don't have a heated bed. Good luck!