As long as there is no space for air to get trapped between the glass and a reflector, it’s no issue to put it on the inside (although its more effective when on the outside) - those foils that can be put on the window using a little bit of water do a good job. Biggest problem are dark objects when only away a few centimeters from the glass - those will crack the window in no time.

At least where i live, non-permanent modifications that use things like suction cups don’t need approval from a landlord.