A tip, buy 2K spray plaster from Hagmans in a can, not a spray can, like a paint bucket. They have it at Mekonomen. It's great to work with and will save you a lot of time and money! I used it during my bachelor project. Straight in soft polyurethane foam without any other plaster and it worked like a charm. You can even apply it with a paintbrush in thick layers and get a good result.
looks nice, are you still planning on producing these?
The sprayplaster you got is, as far as I know, one of the best you can get over the counter in Sweden.
I think the issue mig be that people tend to miss the fact that small scale manufacturing is fairly expensive. If you ask say, 120€ for a non-bolt-on product people will look at other cheaper/easier options. Especially if they don't have a proper garage with good tools/machines. One option I would consider is to offer a to order solution to your fender. In other words, you'll have to drill the holes and sort the mounting brackets for the buyer. Off course this would have to come at an extra cost for the buyer. But I think it might be an option to try out since you don't really lose any money. If you get a lot of response you could get it made as a bolt on product.