I found several cheap foam designs like this racer (I do like it's semi rounded box fuselage) and even easier to build constructions. But these have no (or almost no) airfoil and rely mostly on speed to get lifted. That's not what I need. Some more advanced designs have airfoils, like this boat hull jet.
Still I need something easier to build and fly. More like this trainer one foam design or this blu-beagle design. And that last one is how I discovered the Kline-Fogelman airfoils. Amazingly easy to build using foam sheets! Exactly what I need. So I'll get into construction of a massive wing using one of these airfoils (KFM3 I think).
In case my original MPX Fox fuselage would get to short for my wings I can always get some simple to build box fuselage. First priority is to get better, longer wings. I found some cool looking airplane designs on my search as well. Some sketches:
(check the previous mentioned link for credits and way more inspiring designs)