For an ideal ground you want to find a piece of sturdy metal, the chassis is ideal but if you can't use that a bolt for a seat usually works well. You don't want something flimsy, minimal resistance is the key. Get a grinder or a piece of some coarse sandpaper and sand down the ground spot very good until it is down to bare metal. Secure down the ground tightly using a ring terminal of sorts, and, if you have some, put some silicone around the ring terminal to prevent corrosion
