Stereo & Electronics - Good Ground Point for Rear Mounted Battery?

I'm relocating my battery to the spare tire well in my 94 Formula. I used to have an amp back there and I grounded it to the bolt that used to hold the rear seatbelt in place on top of the rear wheelwell, and it worked just fine. I am wondering if this bolt is adequate for grounding the battery to, or if I should find somewhere else. It's a nice, convenient spot that's inside the car and completely hidden, so it is ideal in my mind as long as it'll make good contact. Thanks in advance.

Cut a hole in the floor, & ground it to the frame substrate under the car.