I wrote about this years ago. If you have a single layer polygon, then it represents the boundary between air (on the camera side) and glass (on the inside). The shader you made is going to treat the entire interior volume of the shower as a glass box.

As others said, if you're going to use any "simple" glass shader, then the IOR has to control both reflections and refractions and the only way you will avoid the massive refractive distortion is to have TWO parallel surfaces, such that the volume of "glass" is very thin. The combined enter/exit refraction of thin plates of glass is just a tiny shift in the light path. The shift through a glass box towards the interior is not tiny at all but enormous.

The other approach that doesn't require geometry changes is to use a shader that handles reflection and refraction (or transparency) separately. Then you can fake the absence of refraction quite easily.

A good example is here:

https://community.hivewire3d.com/threads/good-glass-material-for-poser.4084/

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)