So sorry for the loss of your hens! Smart move to get the rest somewhere else. I've heard weasels will keep coming back. If it had been my girls, they'd be out of there fast.
Hmm, I've been lucky enough never to have your problem. I maybe would put hardware cloth over the floor and a little ways (maybe 6 inches?) off the floor. Make sure there are no gaps, and that the door is secure, as they can squeeze through tiny holes. There are probably other threads on this site as to predator-proofing a coop.
We had mice once. The chickens took care of that problem before I could, lol. I don't think any weasels live around here. They're cute little animals from the pictures I've seen, but man have I heard horrible stories from chicken keepers! Hope you're able to solve the problem.