Now that I've got the goo, I've got to do something with it. Since my original idea was to make something large, I got lucky to find a large roll of cellophane slowly getting old in our supply room. With one piece underneath and another for the top, I sealed it with clear packing tape. I made three different colors of goo and sandwiched it between the cellophane. It ended up the size of our small square table. Now that I write this I wish I'd measured it!
The large squishy bag was passed along to several other teachers before the goo inside got too watery to be fun. Even once the colors were all mixed together into a weird purple after two days, kids still enjoyed it.
While we value letting kids get messy, there are some children who simply don't like the feel of ooey things on their fingers. Activities like squishy bags let them get some of the sensations without the part they dislike. This activity can be done on an easel for a "no mess" day. One class used it on the light table to much delight.
Allergy note: Because the goo is made from cornstarch, this activity is inappropriate for children with a corn allergy. Parents might be willing to risk it if the goo is contained like it is here, but there is always the chance it might leak.