I found this great idea through Teach Preschool on Facebook. The link that Deborah shared for the "Super Foam-erator" originally came from Betz White.
We found that the stronger the plastic bottle the better. If the bottle is too flimsy, it is difficult for the children to use.
We also found that the bottles with a sipper top work best for younger children. The ones without it are more difficult to get a good seal for blowing into.
Have you tried these out? Do you have any additional tips to add? We tried to color some of the bubble soap with food coloring, but the soap was much more powerful than the color and we were only able to produce regular white bubbles.... which were just as much fun anyway!