tray top extra fuel tank .

I have a aluminium water tank off a semi trailer I was going to use as a fuel tank under a trailer but have changed my mind.

Most people just feed the tank exit line (via a suitable tap ie, ball valve) into a T fitted to the breather of the main tank.
You could also incorporate a low micron filter, ie, 11 micron, in the line so any rubbish or grit and water, etc, is trapped before entering the main tank.

I use a 50 litre red poly/plastic tank with a filter, and pump because I have a tub.

If building a plastic tank into a frame on the tray, make sure you leave 2% room for expansion of the tank. The poly tanks expand after their first use and can get 2% bigger. If frame was snug, the tank would become a fixture in the frame and make removal impossible.

These guys have a few options. I was going to get one to mount under the tray between the tray rails as well as a water tank.
I had a 100L water stainless steel tank there in my old ute and it was really handy while being protected and out of the way.

http://www.enmachind.com.au/product-category/poly-rv-accessories/rv-4wd-diesel-tanks/page/2/

I have mounted a poly tank under my tray and have a tap that I have connected via a tee piece to the breather line of the factory tank
I usually just run the main tank until showing empty and then open the valve and it slowly fills the main tank as I drive along
Another easier option is to just have a hose on the poly tank that you can put in the filler cap of your main tank and to top it up whilst having a cuppa on the side of the road

We wanted ours plumbed for both Nick. Wanting an auxiliary tank on our F250, we purchased a 60 litre poly fuel tank made by Dolium, together with drainage hose kit and venting kits to suit. These are made for petrol or diesel in a variety of sizes and were not unduly expensive. We were not wanting much volume, but a safety back up as the F250 has one large fuel tank with no room for a further tank under the tray. If the tank was damaged and we lost all our fuel we would be stuck. So we had the tray tank removable for full use of the tray when home, and had our mechanic plumb the tank so it could be drained into the main tank, or switched to run the vehicle directly from the tray tank.

Nickb
I have a BT50 tray back. I removed the original tank and fitted a Longranger 125 litre tank in its place.
Underneath the tray, I have an 80 l poly water tank between the tray rails.
Cheers
Duncan

We bought a long range tank for our Ranger 4x4 dual cab from Ironman Osborne Park WA. After about 13 months, and beach driving etc not rugged 4wd - too old for that. the baffles had come loose in the tank. Went back to Ironman and was told was not covered. Rubbish. Rang Consumer Affairs - we told them it was supposed to have a 12 warranty and Consumer said - how long did we expect it to last - and we said as it was a fuel tank - longer than 12 months. No go - so we DO NOT recommend Ironman products now.

Got an original single tank if you want to add a tank and link. It's on gumtree

Nick,
I agree with other posters re enmach tanks.
I have one (120 litre) fitted to my 2014 BT50. It's a perfect fit.
There are some considerations to be made, though.

Obviously when fuel is going into the main tank from the aux, air has tl be allowed to escape. There are issues.

The OEM tank breather has a check valve in it, so it only breathes one way, inwards. The check valve is to prevent fuel spillage from the breather in the event of a rollover. It won't let air out, which is what you want, and because of the check valve it is not suitable as a fuel entry point to the main tank.

The filler breather on the main tank is open to atmosphere only when the fuel cap is off, so you cant use that unless you remove the main tank filler cap.

This means that unless you fit a new breather with no obstruction to atmosphere to the main tank, you will always have to remove the main tank fuel cap to allow air out of the main tank to make the transfer to happen.

I have tried joining the main tank filler breather to the aux breather so that both are open to atmosphere via the aux breather, but it doesn't work. Don't know why, logic says it should.

I think the simplest set up for you would be to tee the aux outlet into the main filler tube if you can (difficult, perhaps) or main filler breather down near the main tank (easier) and resign yourself to stopping for half an hour or so so the transfer can happen with the main filler cap off - as others have done.