Be sure to use only 100% cotton fabric, 100% cotton thread and the recommended Warm Tater Batting.
There have been reports that some of these bags have caught fire in the microwave.
I made over 20 of these last year for Christmas gifts. I have had no reports of any fires.
I was given one over three years ago, use it regularly, and have never had a problem.
Have fun at the retreat.
Here's a picture of the ones I made: