I've no idea about the specifics for the dual battery with a Safari snorkel but thought I'd jump in on the battery issue.
Small batteries generally have small capacity, so they won't last forever. Also, a lead-acid battery charged by your car alternator usually won't have much more than 75% charge in it when you turn off your engine. If your battery is a flood cell battery, you'll only be able to use 25% of its rated capacity without damaging the battery (so if that's a 50Ah battery, you can use 12.5Ah). If it's an AGM battery, you'll have 55% of its capacity available. Don't bother with gel batteries, your car will kill it.
Why the difference? Lead acid batteries begin a process called sulphation once the charge level starts to drop. Below 50%, flooded cell batteries sulphate rapidly and the cells begin to degrade. AGM batteries don't suffer this until about 20% of charge.
I recommend that you get your Engel fridge and put a multimeter in Amps mode in series with the power lead and check the fridge to see what it draws. My 60L fridge draws 1.5A when cycling, which is not much at all, but it's an old unit and the newer compressors may be different.
One you have that figure, you'll know how much power you'll need to deliver. Let's say it draws 1.5A - so it will consume that for roughly half the time, so in a 24 hour period your fridge will draw 18A (1.5 * 24/2). 18A is more than the 50A will supply without some damage, so you'd want something a little bigger. I'd suggest for 18A (round it up to 20A) you'd want an 80Ah battery.
If you can fit that in the engine bay, that's great. If you want to run the thing for longer you might have trouble. With a canopy you have the option of putting a much larger unit in the tub.
Hope that helps a bit. There's a lot more discussion about batteries in other threads, including a stickied thread on dual batteries.
How are you isolating the battery from the starter battery?