To all who have replied, I'm going to go out on a ledge and risk looking like a fool...
The targeted vehicle is a Jeep Wrangler JKU with removable fiberglas top. As such, in my opinion, the fiberglas rooftop would not provide any sort of ground plane effect. The antenna proposed (Nagoya NMO-20C) based upon what I could learn/find, is a 1/2 wave, center loaded antenna. With this understanding:
Q1) could I create the necessary ground plane for the proposed antenna?
If yes, would a small circular disc be suitable?
What diameter?
Q2) If the proposed antenna is a 1/2 wave antenna, is a ground plane required?
As you can determine, I'm against the hood mount/ fender mount approach. If there is a lip mount that can be attached to the rear swing gate of a Jeep Wrangler, plse advise.