I am only an spectator on these events, but the rules say you can use the frame from an Production vehicle, OR make Your own as long as you follow the rules etc.
But it is mainly tube chassis cars, more of the frame based cars before. And even if they use a Production frame as a base, it really is a tube chassis With only some "Production frame bits" so to say.
I know one who did this before, and he used an Suzuki 410 type frame as a base. (Samurai in US?)
There are several made on Jeep frames, and atleast one on a Gelendewagen frame, so there is none more used than others I think, when you look at them, NONE of them looks the same.. He..He..
Some similarities now in later years though, but cool With many different ideas and Constructions.