I needed a side draft hood for my forge but the cost was prohibitive. I bought a piece of ac/heat duct that was left over from a job at a local sheet metal shop. The size was 12" x 2 x 32".  I also purchased three separate 24 gauge scraps, 24" x 26" , 24" x 23", & 3" x 31".

Mark the center of the duct and using snips, cut a 12" wide x 10" high opening - I used a dinner plate to scribe an accurate line.

Scribe a line on the back of the duct 14" from the bottom. Mark for one 6" long slot and two 1" slots. Cut with cut-off tool or drill a 1/8" hole and cut with snips. Take the 24 x 23 piece - match the 23" edge to the slots and scribe matching lines. Cut out tabs - the tabs being 1/2" long. Mark the sheet 5-1/2" from the base of the tabs and fold down 90 degrees. ( I used a 2 x 4 and a clamp to make the bend. ) Place the sheet inside the duct, align the tabs, and push the tabs through the slots. Bend down the tabs and pop-rivet in place. Now holding the shelf with one hand, bend sheet down to align with bottom of the duct. Hold with spring clamps. Shape into an arc and secure with pop-rivets.

For the top, use the 3 x 31" - make a 1" deep fold along the 31" dimension. On the fold, cut 1/2" tabs and fold at 90 degrees to the rest of the sheet. On the 24" x 26" piece, cut 1" x 1" squares out of each corner. Mark a 10" circle in the center of the sheet. Start the hole with a chisel and finish with snips. Fold the edge down to make a tray with 1" sides. Curve the 31" piece into a circle, insert through the bottom of the tray, and secure with pop-rivets.

A small hood can be made using the same process. I am using 10" pipes but 8" can be used. In fact, 8" is much cheaper, but 10" had 40% more air flow than 8". Total cost to get rid of 98% of the smoke was $20.00
David Fink