Ground location in bed advice

Looking to ground my bed outlet and from the various threads I understand the rear frame bolt by the rear driver side tire is a good option. There is an existing ground on the top of the frame but wondering if I should do the following:

1. use the hole next to it with a bolt and nut and ground at this location.

2. add an extra nut below the existing ground bolt

3. Or remove the ground bolt, add my terminal to existing and reattach. If it’s this option the nuts are square so is there a special tool for this?

 

the 'square bolts' are all welded studs. don't try to remove them. you'll be in for a very painful repair.

the other side will have the standard hex bolt head.

 

Thanks, realized that after posting. Still tricky to get to that bolt

 

Behind the taillights, there are a couple on each side.

 

I had a look at these but read these are better for low amps. I have 30A circuit with 10awg.