A couple of pieces of plywood, for each window, and some foam strips for air-flow sealing would do. This is not a big effort, just more than your (exceedingly s

A couple of pieces of plywood, for each window, and some foam strips for air-flow sealing would do. This is not a big effort, just more than your (exceedingly short-lived) cardboard design contemplates. The gain in creature-comfort in the apartment, might be more than the money saved in avoided heating, as a renter of the apartment.