Right now, the drawer I've drawn up is 2.25" tall and fits tightly inside the humi. I'm thinking to help with the seal, I'll embed some magnets in the sides (at the front) to keep the drawer tightly closed. As for the top trays, I think I'm going to do two trays, the full width and length of the humi, so 30" wide, 25" deep with dividers to seperate sections. The interior that is not occupied by the drawer is 8" tall, 25" x 30" with 3 dividers at a ten inch gap between them.

Thanks for all the comments and critiques! Keep 'em coming!