HeNe protective box

Hallo!

I have a HeNe laser,9 mW power, which I use on a Michelson
interferometer to measure displacement.

I would like to construct a box around the interferometer,for eye
protection reasons.However I want to be able to see the process through
the box(on the top of it).My question is....

What material should I use in order to protect my eyes and to be able
to see thrrough the box??Does the material of the protective googles
come in thin sheets?

Thank you in advance for your time...

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