Extra fuel is must-have if you prefer fair play without frequent exploitation of the save/load game option. If you take the 5.6 LY hyperjump from Lave to Zaonce: in the regular run of things you have 1.4 LY fuel remaining when you get to Zaonce. But in the case of a misjump you are stranded in the midpoint between the systems and you now need 2.8 LY fuel to return to normal space. You have only 1.4 LY.
Well, we already have an expendable pylon-mounted 3 LY Fuel Tank (author Ramirez) and also some internal fuel tanks (Extra Fuel Tanks & Duplex Fuel Tank, author Smivs). Both approaches are used in real reality – expendable pylon-mounted fuel tanks and conformal fuel tanks. But I personally think that the price difference between the 3 LY expendable Fuel Tank for 75 Cr and the 3 LY internal Reserve Fuel Tank for 50000 Cr is somewhat exaggerated.
My solution: a 5 LY expendable Internal Fuel Tank for 100 Cr. It mounts in the cargo bay and requires 5 tons of free cargo space. It is a single-use primable piece of equipment.
Press Shift-N to select the internal fuel tank and N to activate it. After activation, fuel will be transferred onto the main fuel tank (all extra fuel over the main fuel tank capacity will be lost) and the tank will disassemble, freeing the previously occupied space.
I see the internal fuel tank as a balanced addition to the pylon-mounted fuel tanks. Sometimes one may prefer to sacrifice 5 tons of cargo space for extra security. In this case one can install an internal fuel tank and activate it immediately after entering dangerous system, still having one or two pylon mounted tanks just for emergencies.
Minimal Technology Level to install Internal Fuel Tank: TL8