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Your above enclosure is not secure enough to protect them unfortunately. You need to fortify separate housing for them until they're old enough to be with the big chickens. I don't know how many you have but a sturdy wooden box with an adjoining covered chicken pen would do well, like the one pictured in LaSombra's post. There is no other way to protect them from predators that are still at large.
As to the cat situation, they're going to continue to eat chicks as long as you have chicks that are not protected from them. The only way to eliminate the problem is to set traps and bring the animal to animal cruelty or animal control, otherwise they will continue to come back and attack your chicks.
Cats *usually* do really well with chickens, which is why you haven't had problems before you had chicks, but chicks are a free meal to them unless they're trained, which these are not.
PS: I made the mistake of keeping chicks and keets on my front porch, not well protected and not thinking anything of it because up until that point, I never saw a cat on my property at all. But one night at like 2am, I heard a rustling out on the front porch and opened the door to find a very pretty sable and white cat eating all of them. So unfortunately, unless your porch is inaccessible to cats, whatever chicks you have left will fall prey to anything that has access to that porch. So unfortunatley, fortified protection still applies.