It consists of a piece of vacuum cleaner tubing covered in leather. I've chosen lambskin as it is my favorite garment weight leather, and will conform easily to the corrugations on the hose. I've marked out a rectangle on the grain side of the leather.
I'm making it with the flesh (suede) side out, so I've stitched it the normal way on a regular home sewing machine.
Here I've turned the leather right side out, and soaked it in warm water so that it will stretch slightly, which makes the leather darker.
Here it is half way pulled on. The damp lambskin pulls easily over the tubing. Once dry it will shrink back to its original size, making a snug fit.
The sleeve is completely on now, and even though still damp, you can easily see the corrugations through it. The pressure gauge (which is part of the finished mask) has been inserted to test the fit.
While still in this damp, plastic state, I wrap waxed thread down the length of the hose to emphasize the corrugations. I start by taking two spools of thread and tying their ends together. Then, beginning at the top, I wrap it around the tubing, pulling the thread into the grooves. With the back seam facing me, I cross each row over the seam and proceed working down one corrugation at a time.