Well, what else could I do in this case?
At least, make it as public as possible. Stirr it up at X, Reddit, all the social media shit places available..This would be the best approach, try to get attention, use Twitter and Reddit to ask for help, and be careful, don't make it look like you're accusing anyone of anything, just ask for help and views if you manage to get some interest from a guy with a few thousands followers you might speed it up, Foudnry would rather give it up than ending seen as taking profits from a bug, at the same time, they have distributed those so, it might get tricky.
i think any reasonable company would require it to be taken to court before they say a word about it. it's not a miners responsibility to make sure people don't make mistakes. if that was the case every person that ever got hacked would be taking bitcoin miners to court... 
This is nothing like a hack!
In a usual hack, the miners just process a transaction and the hacker gets the funds, they have nothing they could return!
Here the Op just overpaid something, and the funds have not got anywhere, they were received by the miners themselves by accident.
We have a precedent with Antpool and F2pool so it's about willingness and greed.
The op would win in court easy peasy if he was USA based. But since he is located in Germany the cost of going to court is a lot and as I mentioned I am not sure if this is covered under USA law or German law. It is a shame as this is a simple error which under USA law is required to be refunded.
FOUNDRY IS VERY BIG COMPANY and can afford to return the error. Op should post links to this thread and state the facts.
Maybe on x maybe on Facebook. Maybe on YouTube.