Covering the developer with another tray is the way to go. But not upside down. Float the second tray right side up on top of the developer. It's easy if it is exactly the same kind of tray as the developer tray. By doing that, you are drastically reducing the amount of surface area exposed to air, thereby minimizing oxidation. I do this whenever I take a break of an hour or more.
I posted a comment similar to yours earlier today but it obviously became a fatal victim in the big database crash. I can do nothing but completely agree with you. I have used this method for more than ten years and it works just fine. Sometimes I can keep the chemicals for more than a month. If I am about to make exhibition quality prints I use to mix fresh solutions, just to be safe.
I remember working in a darkroom where the stop bath and developer were left out all the time. At the end of a long day of printing, I'd dump the developer. We'd change the indicator stop bath them it turned purple and the fix when hypo check indicated when we had to dump the fixer.