Oh, and it wont be on the raised beds all the time. I only have those four beds, so I grow in all of them through the whole growing season. The only time they're fallow is during the deep winter, so that's when the chickens will be on there - providing some fertilizer! The rest of their lives it'll be on the lawn, and I'll secure it like any other chicken tractor.
As far as keeping predators from pushing it off the raised beds during the wintertime... I've thought about that. The "Garden Girl"'s fits on the inside of her beds, so she doesn't have that problem. I didn't design mine that way for several reasons, but I think I will drive stakes around the outside to hold it in place. That way the only way it can move is up.
I think I will put hardware cloth over the top, then another layer of tarp over that. Maybe that way I wont have security issues and wont have the replace the tarp every year.
Thanks everyone for all your advice!