My Avion has a similar exterior grill to the one in your picture but in addition there is a three sided cover that screws or rivets to the outside shell , the bottom being open . It looks something like the Ventline without the back flange that would go into the opening . It is also aluminum not plastic . One could probably make one without too much trouble . They were pretty common on older campers , a salvage yard may have one . I'l try to get a better picture if it's not too covered in snow.
And yes the floor can get wet from the frig vent but in truth have only noticed it rarely .
