Students who are absent are expected to make up work in their other classes, band should be no different. I have found with that with this expectation students are way more likely to mark their own sets/practice them on their own time.
Don't sacrifice the morale of the group to catch up the few. Put the accountability on those absent to catch up. Sometimes its better for everyone (including the students who were out) to march a hole for a week until that student meets the standard of the group.
Don't get me wrong, it's not punishment for something out of the student's control, its setting them up to be successful in performance which is the greatest morale booster there is. For some that may mean not marching at halftime until they catch up.