So, that weekend I ran to Costco and got four 10-drawer towers (they were each about $35). Made my drawer numbers and other labels and got the system set-up. Here is a picture of our work boxes. The girls organized the drawers in the color order of their choice.
The numbers are put on the drawers with velcro dots, so as they complete the work in each drawer they move them to a chart and they know when the numbers are all gone they are done with their work. They actually really love this system and are very excited to see what is in each drawer for school in the morning. I usually put a snack in each girls' drawer about half way down, so then it gives them a little break. We do most of the work in the mornings when we can, but sometimes there is work to finish after lunch when Kate (2) is napping. We also do some of our group activities in the afternoon, especially if it is something that Kate isn't old enough to be involved with at some level. If necessary, I include a sticky note in the drawer that says "work with mom" and then they know to work with me for that task. I am always in the school room with them during school time and answer questions and help when needed.
We have been using this system for about two weeks now, so as we keep using it I will update with how it continues to work for us (or any potential problems).