As for the nets, you put them into your quickactionslots on the left of the inventory.
Hoppers will come out of the crevice, even if your standing right next to it (not before it).
If they do, immediately enter combat (not attack, just enter combat)
You can even try to catch them with the box without a net first, if that doesn't work, use the net.
If you miss both, step before the crevice. Wait for the hopper to run away.
Step aside, often(not always) they come back, and often(not always) they don't quite make it, and you can repeat that until your next net is ready.
I'd say a to-hit chance of 50% on the nets is really good already.
If you stand right next to the crevice, sometimes hoppers will immediately turn around and somehow fail, so you have a single try to box or net them before even entering combat.
Obviously, the same approach gives you multiple tried to hit them with something.