I reckon Jet6 had it in his build but was a pic of someone else's as he was looking to do it too.

If you buy a Spiral battery or any sealed AGM or GEL battery they are safe for carrying in the car. they produce no harmful fumes and you can mount them laying down on their side if necessary.

I would be looking at mounting them in the footwell behind the back seat in front of the cargo barrier and drawers if you have them. No problem there.

I read your question to indicate that you had already bought the batteries so you looking to mount 2 x Blue Spirals is that right?

I have also seen one mounted to the back of the engine bay running parallel to the fire wall 90 degrees to the secondary battery directly behind it. A little bit of work but you could move what is in your way there and mount it to the fire wall and brace back to the guard. There is enough room there.