There are actually search results by putting in Ryanair carry on luggage on Amazon.de.
Unfortunately I don't have a recommendation yet as July will be the first time I'll actually max out the space for carry on luggage since they changed sizes a couple of years ago. I've bought one but I won't know how it holds up until the first actual trip next month.I got the 5 cities trolley, but I saw some interesting looking aerolite Ryanair bags if you need some buzzwords for a search.
I will say don't avoid Ryanair and the other very low cost airlines just for the different size restrictions. If you can deal with their other cost saving steps. You'll save up enough on fare that you can easily buy a new carry on should your flimsy one break every couple of trips and still save massively if you are good at the booking game.