We planted a variety of plants, at different stages of growth. After a bit of experimentation, cosmos grow extremely well. These were cuttings we put in just a couple weeks ago, and they're already bigger than most of the other plants that have been in there for over a month.
The very young plants did not survive, but the larger ones did. If they're too small, they just get swallowed down into the cups by the bobbing of the water.
Seems like the plants need to be large enough to reach all the way to the bottom of the pots, and still stick out enough.
Going to be interesting to see what it looks like at the end of the season.
I used 1/2" thick puzzle mat, which was plenty to keep things floating. I used a 3" hole saw, but only cause I had it already (ran it backwards so the teeth wouldn't grab and tear the mat.)
A yoga mat would probably do ok, but I had to add a little bit of weight to the corners to stop the mat from flipping up in the wind (see previous post) so the yoga mat might not be able to support that extra weight.
If you didn't need the cookie cutters anymore, you could probably put a board on top of one and smack it with a hammer to cut circles in the yoga mat, but it also may not work. You'd have to experiment a bit.
I've got hole saw bits, forgot! You can get 1/2" mats cheap enough, but wanted to try what I already have which is thinner.