I have added screen wire to the water heater exhaust vent.I cut the screen to nest on the inside of the vent plus 1" in length and width to allow folding over 1/2" all around to limit wire splinters in fingers, unravelling, and stretching on the bias. I cut strips of 1/2" steel pallet banding with tin snips to lengths which spring or snap into place over the screen in the exhaust window of the water heater. I have had no problem with spider web in the carburation tube since adding the screen.
If you install the bubble screens over furnace
ports, purchase or fabricate the coil spring pusher which makes hooking the retention springs to the grid of holes in the port cover possible. You can make one of these very easily with a piece of wire, a wine cork and a needle file. The wire has to be a light
gauge. Coat hanger is much too large to insert thru the mesh of the bubble cover. Make a V-notch in the spring pushing end with the triangular or knife edge needle file; use the cork as a handle on the opposite end. Only need to be 3-5" long.