Step 1: Optionally, Add a Cap
Step 2: The Players
a 12" (more or less) piece of 3" Schedule 40 aluminum pipe (available from Online Metals, Speedy Metals, McMaster-Carr Industrial Supply, etc.)
One (or two, if you decide to make the optional cap) 2 7/8" fence post caps, available from your local fence supply.
A piece of stainless steel screen, from your local hardware store or industrial supply.
A fitting for your air hose, a washer that fits over the threaded part of the fitting, and a brass pipe coupling that acts like a nut to secure your air hose fitting from the back side.
Not shown here:
a short length of 4" metal dryer hose ducting
a sheetmetal 3" to 4" duct adapter
a 4" duct cap
some duct tape (I used Gorilla Tape)
a couple of 4" hose clamps
Optionally, if you decide to make the top cap, a wooden drawer pull knob and a short screw.
Please, be safe and avoid the use of toxic materials, like zinc or cadmium plated steels.