I'm sure you can get a processing tube for 16X20 film or prints. I used to have one until I got a housekeeper. The cup on the end held less than 500 ml, more like 250, and I'm sure it will do one of your sheets. It's not much of a solution if you are intent on development with inspection, but it will solve your problem of developer freshness. It is easy to mix 250 ml of PC-TEA at a time. I have used such a tube in the past without a mechanical rotator just by rolling it an see-sawing it in my hands. You can also float it in a water bath and keep it spinning by hand.
If you have never used one, don't worry. It's easy. With the tube standing on end, pour the appropriate amount of developer in. It stays in the cup until you turn the tube on its side. Just make sure the emulsion is toward the center of the tube, not against its wall.
I'm not much for development by inspection. It's too easy to be fooled. Some developers start slowly and accelerate, others start with alacrity and slow down. I would rather trust time and temperature, but that's a matter of opinion.