A "snoot" is, effectively, a guide for light, particularly from a flash or studio light. For continuous lighting (eg, as used on a film set) it's necessary to have a snoot made of something which will stand up to the heat. But with still photography electronic flashes (strobes) almost anything reasonably light proof can be used, providing it will hold the appropriate shape. So people make them. (The Makezine Online DIY Snoot appears to be more like a gridspot -- which are better made with black drinking straws (detailed tutorial; another detailed tutorial; examples: 1 2, 3), or with black coroplast (which is corrugated plastic), but can be made with corrugated cardboard at a pinch (something that won't colour the light is better, hence black drinking straws or black coroplast). A grid(spot) will narrow a beam of light even more than a snoot.)
Unlike the Strobist, I don't eat cereal, and have a real job, so I splurged (a whole $1.80) for an (A2) sheet of black posterboard (I also bought a few more sheets of posterboard to use as close up backdrops). After a bit of planning, and measurement, I concluded that the best result would come from using two layers of posterboard, and no layers of gaffer (or duct) tape; I used double sided tape instead. I've made one with two layers of black posterboard, and one with an inner layer of white posterboard and an outer layer of black posterboard (thinking that this would help use more of the light power in the guided area rather than being absorbed by the sides) -- I'm not sure if that's been successful, or if it's just got a broader spread than the all-black one instead.
I wanted to make the snoots a fairly tight fit on the flash head so that they would sit in place with relatively little extra assistance. My Canon Speedlite 430EX II (Canon Quick Guide) measures approximately 60mm across the top and bottom of the head (about 58mm at the top, and 62mm at the bottom), approximately 68mm across the widest point, and approximately 40mm top to bottom. So I settled on make the snoot about 60mm wide and about 44-45mm high, with the final size adjusted by fitting onto the flash head. To get the most robust hold on the flash head I wanted it to sit as far back on the head as possible, so I aimed for about 40mm of the snoot fitting over the flash head.
Construction:
Cut two sections of posterboard so that they are 240mm wide (outer) and 230mm wide (inner) respectively, and 40mm longer than the desired final snoot length (ie, 40mm of overlap plus whatever distance you want in front of the flash head). The width becomes the wrap around the flash, and all folds below are to distribute that width around the flash head.
Create a fold (A) at 40mm in from the edge of the 240mm width section, and a fold at 35mm in from the edge of the 230mm width section. (A common NZ "Taurus" plastic ruler is conveniently 40mm wide, and very useful for folding the posterboard: simply put enough of the board under the ruler, and then fold up hard against the ruler edge, to 90 degrees.) Check that the two pieces sit nicely against each other with the other edges aligned (minor issues can be corrected by trimming one or the other piece; this is not exact science).
Put double sided tape along the edge of the fold (on the wider portion -- the short portion is going to overlap with the other end, and we want it untaped), and also along what will be the front and back edges of the snoot up to about 50mm from the far end (to leave some room for the overlap). Remove the covering from the second side and carefully position the inner section on top of the outer section, using the fold to align the two. Press firmly into place.
Mark and fold 60mm from folded edge A, to form another 90 degree angle (B).
Mark and fold 44mm from folded edge B (in the previous step), to form a third 90 degree angle (C).
Mark and fold 60mm from folded edge C, to form the fourth 90 degree angle.
Tuck the inner and outer layers (hopefully untaped) between each other to form a robust overlapping edge. Use paperclips to temporarily hold them in place.
Test the fit on the flash, and make sure that it will go on relatively easily, but will also sit in place by itself. Adjust fit to suit.
Add double sided tape along the inside edge of the outer section of the overlap. Double check the fit. Then remove the double sided tape covering and press into place. Remove paperclips.
This should work for snoots in the 150mm, 200mm, 300mm sort of lengths without any real adjustment (it's possible to make at least two out of one A2 piece of posterboard). And will work with different coloured inner/outer or the same colour (although with black on black it's a bit harder to see to work; I made the white inner/black outer one first).
The snoot can be profiled by taking shot of the flash against a wall without the snoot, and one against the wall with the snoot, and comparing the two -- that'll tell you how much it is restricting the light (or not). (The snoot should be used -- and tested -- with the flash manually set to its maximum zoom distance: given that you want to focus the light beam you might as well have the flash head doing that too, to maximise the light going where it is useful.) But to meaningfully do that I'll need to get some means of remotely triggering my flash when it is off-camera, as my tests using the flash test button during a 1s exposure in a dark room weren't well enough aimed to do more than indicate the snoot was restricting the light somewhat.