There really isnt room in the engine bay for a second battery but you could probably find room under the rear seat depending on the style of battery you go with.
I plan on putting one together using
this relay (this is a link). It allows a ton of control and can handle high amp loads such as winching. It comes with a remote switch so you can control it right from the drivers seat.
-Manual control switch provides an added level of safety allowing control with or without power, and offering LOCK OFF capability for servicing
-Includes ML-Series Remote Control Contura Switch PN 2146
-Automatically manages the charging of two large battery banks
-Allows paralleling of battery banks for emergency starting
-500 Ampere continuous rating to support high-output alternators
-Magnetic Latch (ML)—ACR draws very low current (<40mA to monitor terminal voltage) in the "ON" or "OFF" states, and draws moderate current for very short time when changing state
-Dual sensing—senses charge on both battery banks
-Start Isolation (SI)—can be configured for temporary isolation of House loads from Engine circuit during engine cranking to protect sensitive electronics
-Engine Isolation (EI)—can be configured for isolation of two engines while both are running to protect engine electronics and maximize alternator output
Senses charging on two battery banks
-LED output to remotely indicate when batteries are combined, isolated, in voltage lockout, or in Start or Engine isolation
They also make some really nice fuse blocks to keep all your accessory wiring tidy.
https://www.bluesea.com/products/category/16/61/Fuse_Blocks/ST_Blade
