I don't think there any wool covers that work "well" with a trifolded pf. One of the biggest benefits of wool is that you only have to wash it if it gets stinky or has poop on it. If the diaper is not secured you are going to get poop on the cover almost everytime your baby poops, and in the first weeks and even months your baby may poop at every feeding and in between. My babies both nursed 3-4 times a night, which I think is fairly average. I had no problem catching every day time poop from about 2 weeks until they started solids, but those early nighttime poops were hard. You'r half asleep and you want them to go back to sleep and.... So...that's a lot of poop and a lot of poop on diaper covers. Which is a lot of work to clean and deal with, opposite of what you are going for doing wool and ec in the first place.
There are plenty of snug fitting wool wrap covers that work for night - Imse Vimse and Nikkis make flannel "workhorse' type covers that are heavy duty enough for nights. They come in velcro and snap, I think, and they fit well enough that you can forgo a snappi or pins. But I would still do a real fold around the baby with the prefold even if you don't fasten it. It's really not complicated at all, only an extra step or two from trifolding, and once you've got it down you can do it in a snap. Even my dh can jellyroll and snappi a pf in under 30 seconds and he swore he'd never change a diaper at all.