When I started I would give them one class period to make up the iRAT. I found that both the students and I spent way too much time and energy making sure they took or extending it if they were sick or hurt. This was all for something that only added 5 - 10 points to their final grade. Since then, shifted my thinking after reading some of Jim's writings. So what is the purpose of the iRAT? We are trying to make sure they did the readings and to prepare them for the T-RAT (the T-RAT is where students demonstrate their knowledge by explaining to their team mates why they chose a particular answer). If a student is absence on the the day of an iRAT/T-RAT (regardless of the reason), they've missed both opportunities.
So to kill two birds with one stone, I established that the only extra credit (or make up for missing an iRAT/T-RAT) is to do an article review. Students have to do research on the application exercise topic. If they do a two page review of a research study (following established format and rubric), they earn the points equivalent to the iRAT/T-RAT (30-45 points). They must complete it within the the unit and are encouraged to share the information with their team.
I use this method because it's the closest thing to a "true make up". They can do. Since they have to research the topic anyway, this merely forces them to do it more carefully and document it. They have to think and write about the topic. Likewise their team benefits from their increased in depth knowledge. As for me I don't mid the extra grading at this level. Surprisingly very few students actually do this. They just take the zero.