yeah, if you have raccoons, then you will want a wire top. No need to get chain link, though. You can buy welded wire at Lowes or a farm store. You would probably only need a small roll of it, which would be about $20. Buy the wire that has smaller squares in it. Nothing is getting through chain link fencing, so you could safely put your tarp over it for protection from the elements. Skunks eat eggs... personally, we've never had a skunk attack a chicken, but maybe that was because they were happy with the eggs. I bet you could read all kinds of skunk stories over on the predator page to see what kind of problems they really do present. This will be a really fun build! I hope you post any progress you have with it.
You might also consider using a second hand plastic playhouse. Put it up on legs and cover the top with hardware cloth. Depending on your climate, they will be be quite cozy during the winter. I live in Augusta, GA and my girls are safe and happy in their run. There are threads on BYC about using playhouses for coops.