I sure agree with Steve, RR. Remember that bats love darkness and heat. You are doing this at the right time to have the bats out for the evening. Pick a warm night to plug the hole.
You might want to check for any other entrances they might use. They can squeeze through some small openings. It might not hurt to check any chimneys for a while also.
And be prepared for an increase in the insect population. Bats make pretty good neighbors as far as keeping the bug population down in an area but I know that you just want them as neighbors, not as house guests.