You could also make one by sticking a vertical dipole inside a length of PVC or ABS pipe that matches your existing vent. The commercial version is nothing more than that anyway. Cost would easily be under $10 and you could cut elements for 2m, 220, 440, etc attached to the same feedpoint for a multiband antenna. As I’m thinking while typing this, I have some 1” wide self-adhesive copper tape that would be excellent for the inside elements. You could make a prototype by sticking the tape on the outside of a pipe and trim to resonance. When the lengths are correct you could make another and stick inside the pipe for the finished version. Anyone got time to make one and publish the lengths for others to duplicate?
prcguy