javajoe79
Fabricator
Well I have no idea about Peru but the easiest way to get a custom tank is to have a welder make it up for you. Here in the States I'd say look at marine tanks; they come in a huge variety of styles to go in the gunwales, under the floor, etc. so there are lots of shapes. The simplest thing would be to have a transfer tank where you pumped from the aux tank into the main but it'd be easy enough to have a gravity feed into your main tank with an solenoid switched valve. you'd have to vent/open the filler on both tanks then close them when the transfer was complete. Again, the marine industry uses these on transfer tanks; the USCG requires a valve at each end of the transfer line to allow for maintenance but with as short a run as you'd have I'd probably put one electric switched valve and one manual (that I left in the open position) just in case you ever had to service or remove the system.
I think he mentioned just using it to fill the stock tank just like he's filling up at a pump. I like that idea. Simple and effective.