I have the harley bolt on side mount plate bracket on my 06 street bob. It has holes to mount the plate vertical or horizontal. I live in missouri and have had no problems here or in Illinois with my plate mounted vertical. H-D bracket is in parts and accessories catalog.
06 street bob. 95" kit, 21" front wheel, wide glide frontend, 17" rear with 180 tire, screamin eagle heavy breather air cleaner, rhinehart pipes, 16" Drag Specialties apes.