Second battery position

I think you can put a small one in the engine compartment - I did it for my Bill's Lux of about the same vintage. 75Ah flooded IIRC. You can fit larger, but it would be overly subject to heat due to lack of air circulation and the tray not strong enough for off highway use.

The tub is a better place for a larger battery.

If you have a canopy, put it in the back. That is where you are most likely to position accessories such as fridge, etc.
I have mine in a battery box which has 12v outlet sockets in cigarette, merit and Anderson connector formats.
I also utilise a dc-dc charger to maintain the battery in the best possible condition and while this method is more expensive, it ensures the battery is fully charged. It provides electrical isolation from the starting battery which is essential. The charger also includes a solar panel input port so I can free camp, maintaining battery charge whilst running accessories.

I have set up a couple of the earlier model hiluxes with an optima red top in an ARB accessory battery holder at the rear of the engine bay and rerouted the starter cables to it and extended the feed to the main fuse box. The optima is lighter than the recomended max weight of the battery tray and has plenty of CCA to do the job.

This leaves the original battery holder that is strong enough to support a 130 AH deep cycle of your choice.

In the 05-15 model the battery cables were the perfect length for this conversion and some of the original conduit mounts line up perfectly with bolts for securing, ends up looking like a factory install. I do not know how the cables in the later model would go, may need to be replaced.
You would also have to check if the later charging voltages will support an optima battery.

Over the years I’ve fitted many auxiliary battery systems under the bonnet of hilux’s and others utes, mainly because they didn’t have canopies (farmers utes with tray backs ). The hilux fits in the top passenger side and several companies make the holders, but it is a tight fit and not much room to work in for the wiring . I agree with sandman ,if you have a canopy put it in the there. I did one for a customer last week used a SSB70ZZD 105 ah 780 cca not a cheap battery but gives both cycling and cranking ability vehicle was an auto so if crank battery was flat he could use this to start. 50 amp Anderson plug to canopy, red arc bcdc1225 with solar control , 220w of solar on roof, 6b&s twin and I use 40 &50 amp circuit breakers as well as fuses recommended by red arc all works very well.

.
Reggie,

By "second battery" I am assuming you refer to an auxiliary deep cycle battery of AGM type.
There is no way that I would position one of these under the bonnet.

After performing some temperature measurements there a while back it explained why my AGM's had a short life. So under the canopy would be my recommendation.

AGM and Lithium batteries and DC-DC chargers all prefer to in a cooler environment.
I no longer put second batteries under the bonnet. Use that little bit of spare space for a second fuel filter or maybe a tyre pump, or just leave it blank!
Also I hate seeing everything crowded in under the bonnet - must be a nightmare for mechanics.

A 2017 lux actually has / had a dealer / factory option for a 2nd battery under bonnet , the 'recommended' battery was an Optima yellow top , no fancy DC-DC required , just a 'smart' solenoid .

Drive a 100 Series so AGM,s under the bonnet has been the choice. Just accepted that it's 3 years worth each replacement. 15 years divided buy 3 =.
O what the hell. It's only money not an arm or a leg.
This time it was suggested I install 2 EXT-Hybrid.There a sealed wet cell recommended for under the bonnet with a 3 year guarantee so will be interesting to see how the go.
Dave.

.
Even if the OEM provided a facility for an additional battery in the engine bay doe not mean that is an acceptable place for an AGM. A flooded cranker or hybrid maybe but not an AGM.
Why should an OEM care if you have to buy a new battery every few years?

In at least some cases the provision of mounting holes may be for engine-bay reconfiguration for steering gear when producing LHD models.

Depending on how long you'll keep the car for, have you considered Lithium? If so, the the canopy is the only place for it with a decent brand DC-DC charger and thick cables.

You need to put it as close as possible to what it's powering.