I was going to suggest the cement mixing trays as well, they come in two sizes, but I can't tell form the pictures if the smaller one would fit or not.
A temporary solution that I've used is cutting the tops off of milk or juice cartons and using one for each cup. It helped until I found a more permanent solution.
Hmmm maybe you can find some plastic storage tubs that would fit? Usually Walmart/Kmart has a big selection of sizes and depths......
I've used their shoebox sized plastics tubs quite often and they're only a dollar a piece!