Big box hardware stores such as Menards, Lowes, and Home Depot carry large trays of HD plastic that go under a washing machine or refrigerator.
36"x36" they have about a 3" depth and sell for about $9. You could buy 3 of them, cut platic out of one to use as a divider for the other 2. Would give you 4 wells. Use a caulk such as 3M weatherstrip adheisive. I don't think it outgasses and is pretty impervious to chemicals. Go the the plumbing section and get a piece of self threading pipe and 3or4 stopcocks and you have an easy way to empty after each session.
Look in the phone book for any restaurant wholesalers near you. They will most likely have a used equipment section were you can buy the already mentioned bussing tubs for $4 or $5. Sometimes you can get lucky and find large 4"deep pans for making rolls that would just fit 16x20. They might be $15 or $20 each but those will last forever. Heck I almost bough a used sandwich grill with the top that hinges down for about 80$. Looks just like a dry mounting press. Mount prints and have a rueben sandwich in the same session!