I had a thread some time back about trunk rust repair, but a lot of the comments are probably relevant to what you have going as well.
Cleaning up floorpans/trunkpan
Seems possible as well. One I found is Atlanta Rotomolding, (404) 328-1004. They had some sample items labeled ‘material hoppers’ which seem similar to a washer bottle. I’m going to guess this kind of business is common in many large markets.
Some quick poking around it seems there are places that will rotomold something for you. I wonder if it’s reasonably priced if you bring your own mold.