I too am planning on putting the radius where the sides join the floor with a router. I found that my sides came past the undersides of the floor when the keyed slots were fit between the sides and the floor. Will be a bit tricky keeping the router bit square but they are pretty close to square as designed.
I will round over the bottom of the center join bar when I flip the camper back over. I am thinking of adding two more bars parallel with the center bar so that the bottom is stiffer and sits across all of the rails in our trailer (We are using the Northern Tool 5′ x 8′). From my initial measurements the camper fits exactly between the fenders. of the trailer. The Engineer in me likes this idea better than attaching on the round pucks that support only in specific spots.