I've seen this done with a 3d lattice of marshmallows connected with toothpicks. It was done without the carousel, so the finished product gives you a nice 3d model of where the microwaves are.
Another neat microwave physics experiment if you have kids, measure the speed of light with a microwave + chocolate bar: http://www.wired.com/geekdad/2010/02/leftover-valentines-cho...
Neat, but it would have been better if he'd not used the carousel.
If you're going to try this at home you should be aware of the dangers of putting water in a clean glass in a microwave oven:
http://www.fda.gov/Radiation-EmittingProducts/ResourcesforYo...
I've seen it done with thermal fax paper and graphite pencil marks.
what's the black bar 2/3rds of the way up?
Blogspam. Here's a proper link - http://kossover.squarespace.com/journal/2010/11/12/seeing-wh... - and it mentions that he borrowed the idea from http://www.youtube.com/watch?v=mcEYvkvfWE8, which appears to be exactly same setup sans the narration.