The downside is that, in order to see the projected image, the room basically has to be pitch freaking black. Especially as the bulb ages and the brightness drops a bit, it's basically impossible to see anything if there are other major light sources affecting the room - like, say, the sun. Now, normally I would solve this by simply not doing anything except at night, but that's a bit annoying and inconvenient.
Usually, my solution is to use a couple of household snack clips to hang a dark blanket on the blinds; this covers about 85% of the window's surface area and drops the light levels enough to at least see the projector image during the day. Unfortunately, it also leaks a lot of light, takes several minutes to get right (blinds, clips, and blankets don't mix very nicely), and deprives me of a perfectly good blanket which I could be using for other things - like sleep. Since putting it up and taking it down is a pain, it also means that I'll often just leave it up for several days running, which deprives me of natural light - and, shockingly enough, that gets annoying to me after a while.
So today I set out to solve the problem, and do it cheaply. I also resolved to do it without leaving the apartment. This is quite the challenge, but being a veteran of two Monkey Island games, two Rex Nebular games, as much of the cracked copy of Full Throttle I could get to run properly, and a Sam and Max game or two, I figured I could probably figure something out.
As it turns out, the puzzle is fairly straightforward to solve, but quite tedious.
Start by gathering up 5 black plastic garbage bags - the thick, heavy kind like you would use for yard trash. Locate a roll of Scotch tape and three paperclips. You may or may not have to have a drinking contest with a sasquatch to acquire the third paperclip, depending on your selected difficulty level.
Once you have gathered the materials, measure out the window area, and lay out four of the five trash bags in a way that will cover the area completely. Real Men don't use pansy tools like measuring tape or rulers, so do this largely by holding up the sacks in front of the window, squinting one eye, and muttering "hmm... yep... should do."
After finding a suitable arrangement of the bags (they will probably overlap a bit), tape them all together. Everyone knows that a Good Engineer will spend an inordinate amount of time taping in bizarre criss-cross patterns as "structural reinforcement." Consume a shocking quantity of tape for this step.
Take the three paperclips and bend them into hooks: unfold the outer loop of the clip and bend it straight so that the inner loop remains, and there is just a line of wire running straight out from the loop. Then fold over the end of that line to form a hook, leaving part of the inner loop intact - you will have a double-ended hook type thing at this point. Twist the outer hook 90 degrees so that it lies in a plane perpendicular to the inner loop/hook.
Now, hold up the sheet-o-bags to the window, and mark off points to attach the paperclips. Tape the paperclips' inner loops to the sheet-o-bags, and use the outer hook to suspend the contraption on the blinds.
Note that attaching the hooks to the sheet is tricky - you can't just slap tape over it or the weight will be too much and the sheet will just drop off. You have to create a little "pouch" of tape (with the opening pointing down) and actually insert the hook into that pouch so it catches; then tape over the whole mess a bunch of times so it's sturdy. Looks like crap, but can hold quite a bit of weight.
Finally, suspend the sheet-o-bags-n'-hooks on the window. Slice up the last bag with a knife and tape random squares of it over the contraption to patch up any thin spots where light still leaks in. Apply another shocking amount of tape here, just to be safe.
And there you have it - a custom folding window cover, for less than $8.
The room is now almost utterly pitch black with the cover up - it's slightly oversized so it blocks out the entire window area except for a tiny sliver over the top of the blinds, which doesn't let in enough light to really matter. In the worst case a little strip of felt can be shoved up into the crack to totally seal it off, but I doubt I'll bother getting that hardcore.
The best part is, the whole thing takes about 20 seconds to unfold and hang, and maybe 15 to take down - so it's far more convenient than the blanket-hack solution. It also no longer monopolizes a blanket... yayy!