Having owned one for 40m and 80m all I can say is they were both a coil of wire in between two aluminum plates. Look at the pictures on the website and break out the resonant frequency formulas... shouldn't be too hard to come up with a "workable" design. I'll not go into how good they do or do not work, only say that in the absense of available space they do work better than nothing and will get someone on the air. I only considered them for those 2 bands as I had enough roof space to put up a dipole for 14 through 30 mhz and dipoles do work well.
Never do anything you don't want to explain to the ambulance technicians.