Just a brief bit of research yielded:

Lowes sells 10' x 20" aluminum flashing. Its thin material and you can use a standard paper hole puncher to punch for rivets.



Circumference = Pi x Diameter

Ex. 14" Grill top diameter requires about 44" of the rolled up flashing. You can get two smokestacks, 38 or so inches tall.



You can use a yard stick as a guide and a cheap razor knife to cut it. Its lightweight, but can very easily be reinforced. If you want, and at the cost of a little material, you can reinforce it to hold a tank.



If you "bend" it, you can clamp it between two pieces of wood..... again, a little research on a redneck bending brake...



I have a son who is allergice to milk, soy, peanuts, and eggs. I make him biscuits, pancakes, cobblers and all kinds of things that he would otherwise not get to experience in life if it was left to commercialization products only. Plus i love to go camping. Through instructables, ive made aclohol coke can burners, Joule thief and fuji camera lights, solar battery as well as peltier devices that, when placed over a cup of boiling water, will keep some leds running for a long time with the simple burning of a tea light candle. I have a weber, table top grill not unlike the one in your instructable. I keep all my cooking supplies inside.



An additional idea or two.

Make a spit:

Rivet a couple of washers, opposite from each other near the top of your smoker. Then devise a "spit". Second hand stores sell oscillating fans for 3 or 4 dollars. The small electric motor generally has a 5 or 6 rpm rating. Use a counter weight to offset the amount of torque required to turn your spit, and it would expand your options. Most come with a capacitor, and through experimentation or research you will have a really cool conversation piece :)



Make it collapsible:

It may not interest you, but by thinking out of the box, other people doing other things might would benefit if you were able to "undo" the stack, then re-roll them back up.



Make it shorter by half and you have the ability to "roast" things at a temperature above 250 deg F. I use open pit cooking to cook briskets, butts and shoulders, but as Charlie Daniels would say, "boy lemme tell you what!", if you take a big ole ribeye thats been covered with your favorite concoction, get you a stoking fire and that smoke stack will trap the heat around that steak and the top will be open. There is a differnce between cooking on the grill top and cooking with the meat an inch or two below the surface. I dont like cooking steaks "covered". I like my meat to be marbelized and not the same temp all the way through.