Look hard and some come up for reasonable money. I agree that they are well made too. The washers seem a whole lot simpler and rather expensive for what they are. My quad 20x16 was about £250 used and is the last thing I would do without in my darkroom.
Whilst I have made a great big drying rack (quite easy), I think (having looked into it) making a big washer is beyond me. I suppose they end up under bit of pressure too when the water flows.
For those in the UK, I found a company (sorry deatils I cannot remember) called something like UK or British plastics based in the Birkenhead area. They had everything you could imagine in plastic and I got some diffusion material from them for a few quid. They would certainly have what it takes for a British DIY project.