So I made a mob grinder. Really basic, just going off the usual canal structure, and pretty small too. (Though I can flip it into an xp grinder easily too.) Just one cross section (which makes four platforms of 8x8). Built it up above the ground because I really just wanted to get rid of a lot of dirt (and be able to see what I was doing).
I thought I did it right, but I hung around for easily more than a half hour (including an overnight) and nothing spawned in it. I even got the ambiance when hanging around (the little "dark area nearby!" sound). A creeper did spawn on a nearby island overnight (I made it out in a small cluster of islands); would it be worthwhile to kill the passive mobs there and terraform so there's fewer places for mobs to spawn, or am I going to need a bigger farm no matter what?