3 rd battery in a GU patrol 3.0 TD engine bay??

The GU 2.8 actually uses that space for the second battery. You may be able to the 2.8 tray there. I know it won't fit in the later GU's with ABS as the ABS gear is located there, but if yours is wthout, you may be able to squeeze it in (above the sink)

Cheers

I have seen one fitted there in a GQ, had to move a bit of stuff around tho

Maybe you should just buy a power pack and they can be put anywhere within the vehicle and even recharge from your cig lighter or aux battery whilst driving.
Thats what I have done so that the power pack can be used anywhere to provide light or power for a portable air and water pump plus many other applications.

I have a GU Patrol and have put a duel battery tray up against the fire wall on the passenger side where you are talking about. I bought it second hand but looks like it was professionally made. It has an opposite lock sticker on it so wether it was made by them or not I dont know.

My 4by does not have ABS so it went in that corner nicely. All I had to do was
re-route the plumming for those air tanks and screw the larger tank to the battery tray. There is also 3 brackets that bolt down into excisting weld nuts and 1 that I had to put a nut and bolt through. That one went through the lip where the fire wall joins.

The battery tray has a side on it that replaces the bracket for those air tanks if you get my drift

Not sure if it would be any different as I havent seen under the bonnet of a series 4 3.0 but my Patrol is a 98 4.2 diesel

It seems to be pretty solid when I give it a shake

Hope this helps
Cheers Luke