Hi. The plans are based on a patented device manufactured by ASC(Acoustic Science Corp), in Eugene Ore. I've visited their plant and saw how these are really made, and they are not easy...nor cheap. The tubular rigid fiberglass is especially expensive, and comes in short lengths, which must be glued together. Then they are wrapped in a metal screen, which is fastened the MDF endcaps to hold the thing together, of which also gives a substrate to fasten the fabric to. Prior to slipping the fabric sleeve over the tube, they are wrapped in a clear Mylar sheet with 2" perforations, which acts a diffuser(or so says the inventor..who is also the owner of ASC). These have been around for a few decades, and lot's of people swore by them for years. At the time of their invention, they were at the top of Acoustic product accolades. But that was then.
However, in this day and age, there have been many advancements and Lab tests of much simpler ideas. The Superchunk is one such idea, proved by Lab tests to perform better than other possible "DIY LF absorption strategies. Do yourself a favor and forget these Tubetraps. Invest what budget you can in plain ole 4" thick rigid fiberglass panels. Knauf sells this product in 4'x10' panels, which can be easily cut with an electric knife or skill/table saw. There are tons of information on building corner " bass traps/Superchunks on this site and all over the Internet. Just do a search. In regards to the Plastic sheet..it's purpose is to reflect Hi Freq sounds so the room doesn't become dead. The purpose of putting them in the corners is simple. This is where Low Frequency sound waves terminate, and reflect from. Hence the term Bass traps...which actually is a misnomer. But I won't go into why. However, in small rooms, you need morethan just bass traps in the corners. Broadband absorption panels(2"-4" thick with an equal airgap behind them)distributed on the walls and ceiling in various patterns breakup specular reflections, add to the absorption coefficient via the Edge effect, and offer a modicum of diffusion as well.
However, in case you still want to build the tube traps, here's a couple of Sketchups I did a while back.
For clarity sake, here is one with the fiberglass removed so you can see the screen.
And here is the fiberglass withOUT the screen, but WITH the perforated Mylar. And btw, regular ole 3 or 4 mil plastic sheeting works just as good, and on Superchunks too.
good luck what ever you do.