I used 1/4"x20 x 1/2" long threaded button head bolts and 1/4x20 T-nuts to bolt it to my battery box.
You also need a piece of aluminum flat stock about 1/4" thick.

Drill 2 holes in the battery box big enough for the T-nuts, then through the bag, then the flat stock.

The T-nuts go inside the battery box, they are the flushest way to go in there.
Run the button heads through the flat stock, then the bag, then bolt to T-nuts.

Pretty simple 15 minute job.

 

I used the existing 4 holes that were there, drilled holes into the bag, 4 bolts with large flat washers on the inside, the bag is very thick stiff leather so I didnt need any back plate like some do.

 

What's the difference between large flat washers and a back bar?

They serve the same purpose, to keep the bolt heads from being pulled through the leather, regardless of how thick your leather is, it will happen over time if you don't give it a bigger surface to hold onto..

 

Lots of people us a back plate to keep the shape of the bag, so thats the difference, of course the washers prevent pull through, but because the leather on my bag is stiff a back plating is not needed. Clear as mud?

 

Flat stock is not necessarily just for keeping shape, when using t-nuts to keep it as flush as possible inside the battery box a little extra thickness is needed, that is why I said 1/4" thick, I guess for you I should have said a 1" wide, 4" long by 1/4" thick flat stock.

You sure are hung up on the thick leather

 

Rite on, thanks to the both of you, I cannot wait to try it!