I worked in a commercial B&W lab and the only thing we kept is the Fixer. It was the last tray and it was the biggest. We put a cover on it at the end of the day. All the printers needed to make a big batch of developer when they come in the morning and pass it along the other printing rooms to dilute 1:2. Stop was plain H20.
I did what you did at a home garage darkroom but I put plastic right on top of the chemicals. Oxidization occurs...when chems hit oxygen. So minimize the oxygen by covering right on top.
Well, that got messy and it was okay for a month or 2. So I bit the bullet and got an Nova vertical. They are worth every penny and the darkroom footprint comes out so much smaller.