Witch Hazel commented on the last post.
Interesting idea. Duvet cover fabric might not be stiff enough, but I guess you could buy some iron-on interfacing (maybe instead of the fleece? Although "as well as" the fleece might also be an option) to give them more body?
For light blocking, maybe use some blackout blind type fabric - you could probably sew this to the inside at the top. Maybe you could use this instead of interfacing to stiffen as well? Not sure how stiff it is.
That has given me some inspiration. I know that you can get wonderweb stuff so you iron two pieces of fabric together and they stick. Or I could turn the duvet inside out, iron on the interfacing (thank you) and glue on the fleece and then turn right side out and catch stitch in the right places.
I had been caught up wondering how on earth I could sew fleece on from the wrong side without catching it or wrinkling it up. To be honest I am not too worried about how stiff the fabric is as I just want to stop the neighbours getting shocked.
It is still just a thought experiment, but I've started putting together a raggy throw I had half started some months ago. Who knows, I may eventually try the duvet/curtain out. If I do, I shall share!