Hi,
I have done exactly this, and it has worked great.
I just brought an electric screen of ebay, cut a rectangle large enough for the actual screen fabric to fit through with space on either side.
Screw the screen to the wooden joist (this was easy for me as i just went into the loft and screwed it from up there.
To make it neat i just used a bit of thin wooden sheat , cut it into a rectangle and then cut a nice neat smaller rectangle in it for the projector screen to go through.
I probably could make this sound much simpler but im not very good at explaining some things

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My Home Cinema Room / Bedroom
(Link to a previous post with pics of my setup)