How to make a sensor opening waterproof

Hello,

Not really a Kiel related question, but I was hoping somebody might have an idea. I have an IR sensor connected to a microcontroller, which is part of a system that should withstand harsh weather conditions including rain. I have drilled a hole for the sensor in the plastic casing, but water will no doubt leak into the product from the edges of the IR sensor once mounted. I am reluctant to use silicone to make the hole waterproof out of fear it will interact with the plastic cap of the IR sensor (are those fears founded?). Do you have any idea how to seal it?
Thanks