To be perfectly clear about this screen, it's a fantastic alternative to buying a screen for $300. This was my "bandaid" screen until I got a real screen. I'm perfectly happy with it. Real ones reflect back from the crystals. This isn't that.
- I had to cut mine down a few inches so it's not 100" now. (An aspect ratio thing)

I've been using a projector for a few years now and projecting always looks bad on bed sheets, 40" press paper, wax paper and yes, even white walls. I needed something big, cheap, and light. My first attempt was with a 4'x3' .25" sheet of plywood that was super heavy and expensive. Here is what I did, and how I did it. I'll go through the easy steps. The only time consuming part is the painting

Here is a list of what you will need.
1. Paint - Make sure to use latex as I first tried some Krylon spray paint and it melted the foam. One quart of Behr "Silver Screen" (it's light gray - yes light gray). It's normal latex paint found at Home Depot I believe. If you can get a better deal on another brand just use the paint chip to have them match it. If you can't find it just get light grey. Buy flat or eggshell. No semi-gloss or gloss. Using matte/ flat hides most imperfections.

2. One, 4' x 8' x 2" (actually 1.5" thick) sheet of Pink Panther (or other?) ridged Styrofoam insulation . Look for the smooth stuff and pick one out without any dents. Some is stamped on one side and smooth on the other.

3. optional - Four, 1/2" x 2.5" bolts , 8 nuts and 8 big washers. (see "Hanging It" section)

If anyone does this, will you post a picture on this instructable? I'd love to see it, and see what you changed.

Step 1: Lights on - Scale - Frame of Reference

As a frame of reference, see the picture with the lights on. My screen is angled as it's in the corner. It's also angled down as my projector is on my desk. Everyone mounts it differently so disregard this. The picures are more to show the scale rather than how i mounted it.
Back-lighting is mentioned later as well.

Step 2: Size Considerations / Cutting

My best advice I can give you on size is to set up your projector and take your projector mesurments before you cut. I'd recommend setting up the Pink Panther styrofoam where you want it. See what you'll want to cut. You need to decide what aspect ratio you want. 16:9 , 4:3 ++. I'd recommend 16:9. With a 4'x8' sheet the biggest you can go with 16:9 is 4 'x 7.11'. Here's a link to a ratio calculator.
http://www.wideopendoors.net/design/aspect_ratio_calculator.html
http://tvcalculator.com/

Black Border
I didn't do this on mine, but people also like a black strip around the border to clean up the edges of your projector. Consider that with your measurements as well. I cut off 6" (I made mine 16:10 and wouldn't do that again). I used a hand saw lightly and that worked well. Or razor and snap.

Step 3: Bolts for Hanging - Optional

This part is optional .
The bolts (with large washers) are needed as you can't really screw hooks into Styrofoam. You can, and I did, but they came out. I did however add a bunch of duct tape and glue in the hole. That has held for my indoor screen but won't for outside with any wind. Not recommended. I'm sure people smarter than me could figure out a better way, so I'm open to suggestions.

These pictures are of the screen for my deck (V2). I drilled them and installed the bolts and washers before painting . I used bigger washers and on the back (below) added another nut to give me a gap to hang on a lip or add rope/ wire. If your going to flush mount it on a wall with velcro or something, then skip this step. I used these on all four corners so I can flip it up to the ceiling of my deck when I'm not watching it.

Don't get too wound up about the appearance of the screw heads on the front side. You do this step before you paint so it will be camouflaged by the paint. Nothing good happens in the corners anyway right? Seriously though, you really can't tell when it's painted. Go in at least 3 inches top and side or you'll regret it when your redoing it because it broke off. It's still Styrofoam and can be a sail in the wind.

Step 4: Fill Slits - Painting

The Pink Panther Styrofoam comes with their logos on one side and there are small slits and smooth on the other side. Thats your painting side. (easier to snap off at a jobsite?). I left mine and just added more paint but you may want to fill it in and sand. Flat paint hides seams better so see what you think.

Get the Behr "silver screen" light grey paint out and paint it. I rolled mine on with 3 coats all the way around front, back, and sides. Why the back? Light comes through it easier. More on that in step 5.

The paint almost doesn't look grey. The grey brings out the blacks and contrast. Projecting onto white makes it look blue or fluorescent or like 6000k white. Grey seems counter intuitive but that's why movies played on the "silver screen" back in the day. A grey screen solves most of your color problems that you WILL have projecting on white. Behr's Silver Screen is normal paint. If you can get a better deal on another brand just use the paint chip to have them match it. Buy flat or eggshell. No semi-gloss or gloss. Using matte/ flat hides most imperfections as apposed to any other type. I did eggshell and I can notice my roller lines once in a while.
I took some steel wool to it before my last coat. I tried not to go overboard as this is a quick and cheap screen. In a perfect world I also would have sprayed it instead of rolling it. It doesn't seem to make a difference with flat paint but I can see roller lines on my eggshell version. Just a heads up. I'd consider semigloss with a paint sprayer I think, but not a roller. Semi gloss looks great on your wall but you don't shine a bright light on it and look at it all day either now do you?

As a side note - The term "Silver Screen" started in the early days of the motion picture industry. Screens were embedded with reflective aluminum because projectors back then were so dim. The silver (aluminum) reflects back to the audience better.

Step 5: LED Back Lighting, Backlit (optional of Course)

Optional.
I love back lighting because I'm a weirdo and I think it looks cool. In my case the screen is in front of 1 window so I hooked up a photocell dusk to dawnplug in for it. The ones you find at Target around the Christmas tree section. Pops on at night. I have 2" of clearance on top and 2 on the left side. I originally mounted the lights to the back of the screen but the light came through. Even with 3 coats on the front and back. Light also came through from the window, and washes out your Color. Don't throw in the towel on this screen if you tried it during the day and wasn't properly blacked out on the back. My solution was to cover the back of the screen with space blankets (Target camping section) . Tape it or use push pins. You could also black out the back, but I want the light, just not through the screen.
This screen has 3 hooks in the back (my indoor one). I wouldn't recommend that though. I had to glue all of them in. My screen is angled down so I needed them there.
The green border was meant to illustrate the back of the screen. It's hard to take a photo back there. Check back.
Depending on where you put the screen, you don't have to get too tidy on the back. You'll never see it.

Step 6: Random Notes - Upload Your Version

Random notes:

--I poured a few ounces of the paint into triple sealed sandwich bags. I've had it rained on, blown down (deck) and scratched. Dip a napkin in the paint for little touch ups. I'd rather wreck this screen than a $200 version.

There is nothing worse then a paranoid "projector guy" who needs to take everything down hours before it rains - then never does. Figure out your exit strategy for your projector, speakers and computer, not your screen. A rolling cart into the garage always works well.

I run all the video and audio from inside and pipe it outside via a "Cat5 to VGA" adapter and an FM transmitter (also inside).
Run that computer with your phone or laptop with LogMeIn. That way you grab your projector, radio, and 2 wires. 30 second tear-down. Let your projector cool down if at all possible. Have a weather radar and an umbrella.

See if you could project from an inside window and cover all your bases. It's never that easy though is it?

--I'd also like to do a version of this with hinges to fold into 4 sections to make it 2'x4' x 4"(?) for an outdoor version. It's light, but it's still big.

So again. This is great because it's cheap, easy and works pretty well. It works great outside too. You can add a hook to your house or garage and hang it like my deck version. Watching a movie on a sheet blowing in the wind seriously gives you a weird motion sickness kind of thing. OK. Maybe not sick, but it messes with your senses.

If you made one, please upload it.