What is a good material to block all light from a window with?

Blackout curtains have always disappointed me because they let in far too much light. Both through the fabric itself, and around the edges.

I would like to create a window covering for my bedroom that doesn't allow any light through. Such that I can't differentiate the window from the rest of the wall when it's covered and the room is dark.

My best idea currently is to use plywood and flush mount them with a silicone or rubber lining around the edges to further seal it. They'd open/close like this:

But I'm worried about the weight of the plywood making it unwieldy to open, and dangerous should it fall. I'm also unsure of the thickness I'd need the plywood to be to both be stable and properly block the light.

Does anyone have ideas for a better material?

An answer to a similar question suggested rigid foam, but I'm pretty sure that foam wouldn't be stable enough for years of use.

little money, can't afford central air and heating, I truly appreciate and thank you for this post and so will family and friends. Sincerely."

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"Great ideas, simple, that are totally affordable and saved me from this drastic heat. I am a senior citizen with