In order to get useful brightness, a lens must be used. Here another problem crops up--the focal length needs to be equal to the distance from the wall with the lens to the back imaging surface (just as in any "camera"). One can make a simple, long focal length lens by combining a negative and positive lens. For example, a +2.5 diopter (0.4meter focal length) positive lens combined with a -2.0 diopter negative lens will yield a +0.5 diopter power, or a 2 meter focal length.
A panel of foamcore or similar white material placed 2 meters from a blackened window with this example lens in it should provide a good viewing surface. Black plastic sheeting used for protection of floors when painting is pretty opaque, and is good for blacking out windows.
Be prepared for dim images. Suppose the lens diameter in the example is
25mm, yielding an f/no. of f/80. The "brightness" of the image on the
screen will be:
B(i) = R* B(s)/(4*f^2)
where
R = Reflectance of the image board (approx. 0.8)
B(i) = image brightness
B(s) = scene brightness
f = lens f/no (80 in the example)
For this case, then, the image will be 32,000 times dimmer than the outside world. If the outside is reflecting an average of 3x10^4 lumens/m^2 (midday bright sunlight), about 1 lumen/m^2 will be reflected from the white panel. This is about 10 times the apparent brightness of a scene lit by the full moon at zenith.
The advantages of a small room (camera) are obvious from the equation. The image brightness will increase by a factor of 4 if the dimension is reduced to 1 meter, or decrease by a factor of 4 if the dimension is increased to 4 meters. If you can find a larger aperture lens, the brightness will also benefit by the square of the lens diameter.
John H. Hammond "The Camera Obscura" (Adam Hilger, Bristol; 1981) - a nice little book on the history and existing camera obscura for public viewing in about 1980.
Book three of the series "Amateur Telescope Making" (1956) has a couple of pages on what they call a camera oabcura, (which is sort of a large view camera) but describes the use of a simple meniscus lens with a stop in front of it (described as a rear landscape lens). The relatively wide field of view of the landscape configuration is a plus in this application.