I don't know the exact answer to your question, but I can point you to a good place to start...
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recently did a simple front-projection screen based on info from there.
At 6x8 feet, I can imagine it will be difficult to keep the material from flapping about, resulting in image distortion. Is rear projection necessary? Front projection onto a sheet of painted MDF is cheap and easy.
Start smoking and use the inner tissue paper liner that has a layer of foil bonded to it. With effort you will be able to separate the two. When you have several thousand of these you can glue them together to make a large screen.
Rear projection is a requirement, as I already have a 'sister' rear projection screen and wish to construct another. The first one however is a theatre grade frosted polyurathane type of material which is velcro'd to a steel frame. (read mega ££££'s)
The 2nd screen i wish to construct could easily be made to look near enough identical, certainly from the audiences point of view and the low light conditions it will be used in.
The brief is really to make a 2nd screen of simialr size to the first which will be used the other side of a DJ rig (symetrical)
Depending on the size you might be able to use a plain white shower curtain. It's technically possible to join them (using 2" wide clear tape - the waterproof sort sticks well) as the edges are cut nice and straight.
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