Hi New Guy,
There is very little space under the forward arms of the horse shoe cover and the plastic is there to protect the soft top material, which would be in direct contact with the cover, without the vinyl protection. The vinyl sheet is not as thick as 15 mm. I would hazard a guess of 2 to 3 mm. My hard top is on the car, so I can’t have a quick look, sorry!
Regards
Chris