Stana Katic (if she wasn't already otherwise occupied with Castle
) would make a great Lois. She's a few years older than Cavill, but she tends to be a bit of a chameleon and would easily make it work.
Missy Peregrym, Rachel Bilson, and Mila Kunis might be decent choices, too.
(Years ago I liked Rena Sofer for the role, but she's 42 now against Cavill's 27. That's not going to happen.)