Light proofing for windows

[BFG Central]

I was giving some thought to how i would hide the fact that i had electrical power if the SHTF and first i went down the route of black out curtains and heavy linings.
But i tried that on a window and no matter what i did, even the light from a head torch came through the gaps at the top.

So this got me looking for more options and after searching around i found something called white/black/white plastic sheeting.

They do make a black and white sheeting but some light gets through that.

Its actually sold for hydroponic growing to block and reflect light.

It comes in a 2 or 3 meter wide roles so it will cover almost any size window.
It would also double as window sheeting in the case of a pandemic.
Add an extra few roles of gaffer tape to seal around the windows and doors.

Has anyone else got any ideas on this?