A cross border rental will incur really painful one way fees. The rental company has to get the car back to the original country - having the car re registered in the destination country is a surprisingly difficult and bureaucratic process - even within member states of the EU.
I would rethink the itinerary. If you really need a car, plan on dropping the car in same country in a town near the border and crossing by train to the next country and pick up a different car.