Johannes, back in the day I used to pack both trunks - like a puzzle - to go on long road/camping trips with 2 people, and always found the 16 cf of the 914's 2 trunks more than adequate!
Maybe I was just a good packer?
Also managed packing for 4 (wife & 2 kids) for 4-6000+ mile 2-6 week cross-country (USA) trips in our 88 Westy!
This is good for the "trunk challenged" MGB, Miata, S2000, etc. crowd though!
Cool idea, but I'd want to make sure it didn't scratch up the paint from vibration/movement while driving on though trips.