My Versalab 11x14 washer can do two 20x16s. I remove the divider rack and put in two 'U's of flexible acrylic sheet. If you only have space for 2-3 trays you could likely skip a stop phase and go straight to fix. At this size the paper is the major cost, not the chemical capacity.
I have used Jobo print tanks on a hand roller base for doing 20x16. Although drying the tank is a pain, for a couple of prints it is less trouble than reorganizing for the large trays. I use 8x10 trays for tests.