I have 2 sunfish and can say that they fit perfectly inside with the rear seat down. Sure they hang out the back a little, but the width is perfect! I rescued 2 and brought them home in 2 wagons. Just be careful with the masts, the guy who gave them to me was 'helping' load and jammed the mast in and smashed the AC vent by the passenger door (a hard part to find on a TBI Chevy, car was a 92).
I drove home on the interstate at 110 kmh with no issues.
BTW where near Amsterdam do you live? I used to go there for work in Zoetermeer, and also stay in town in the Leidsplein area.