IMO the Braille batteries are probably your best bet as far as easily concealable. I’m sure there are other brands, but Braille is who I usually think of.Your options with a full sized battery are limited if you want it hidden. Maybe you can fit on in the spare tire well? I did some research about this and read an article where the author moved the battery around with the car on corner scales. It made a big difference, so that may be sometime to consider. Here’s the link:
The Hows And Whys of Battery Relocation - Garage Tech
Once just for better weight distribution, battery relocation has also found a home among show car builders looking to clean up an engine bay. We show you how to do it right.www.motortrend.com
I too recently stumbled across brail batteries. I think they would work for me. Maybe when the AGM I got last year dies I will try one.
I wouldn’t even consider trying to mount a standard 50lb battery somewhere weird.