Sometimes the best in-car GoPro location is on the diagonal bar off the B-pillar hoop. This simple OpenSCAD design is for a 1.75" bar diameter at 45-degrees from the horizontal. Printed horizontally on a MakerBot 2 with seven shells, 50% infill, support material, and a raft. Attach to bar with hose clamps, as shown. This race proven design provides a stable camera platform!!!
See https://www.thingiverse.com/thing:2883682 for another mounting option.