Originally Posted by
YVR Cockroach
Not entirely sure where you're from but if you can use a European website or car rental broker, start there. There's a lot of European one-way traffic on that route so there's arrangement with rental companies to balance traffic.
+1. I have done this drive (although not in super peak season like you want to) one-way and found decent rates on the German version of major car rental agencies' websites.