Originally, I designed my darkroom just to print 12x16 FB papers, so I made a wood sink (175 cm x 65 cm, inside measures) that could keep four trays inside it, and with a Nova Washer everything was fine.
However, when I wanted to print 16x20, it was a little bit uncomfortable since I could only work with three trays.
Finally, I solve this problem making two shelves leaned on both sites of the sink. Now, I can work with five 16x20 trays comfortably. It is not perfect, but it is much better.
As a marginal comment, I want to recommend you to buy a washer big enough. I bought a 12x16 Nova and it is very good for 12x16 FB papers; however, if I make 16x20, I always should print with RC papers, because it is not easy to wash FB papers of this size without a washer.
I am annexing some photos showing how the shelves make me possible to work with five 16x20 trays. I hope this idea could help to those people that, like me, have a small darkroom.