I think a cage would be unsightly IMO
. An intake strainer
is usually more then adequate for keeping fish/inverts out and they make one to fit just about any power head.
If you want total peace of mind a sponge prefilter
will keep everything out. Personally I like these since my shrimp will jump on and get all the little food particles that stick to it. Keep in mind if using sponges that they need to be rinsed off when doing pwc
in your old SW