I'm also an ectomorph, so gaining mass for me is pretty hard. But right now my butt and thigh are pretty thick, that my mom thought I was using pads when I was using compression shorts lol.
And the only leg workout I've been doing is explosive pistols, 3 x 10 on each leg, and some minor workouts that lightly uses legs. But of course when I go full time I'll implement more exercises into my regimen.
P.s. just be careful with pistols, make sure you can do them properly, before trying, or you could seriously hurt your ankle, knee, or other.