A must on any flight box:
Fuel Gallon Holder, Power Panel with fuel pump/ glow igniter/starter connections, 12v batt, starter, electric fuel pump,
tachometer, drawer for misc tools, glow plug igniter. In the drawer I always keep allen wrenches, a pair of long nose pliers, adjustable wrench, screw drivers, glow plugs, CA adh, zip ties, glow plug wrench, control horns, servo arms, clevises, etc. Also extra props and baby wipes (they are excellent for cleaning your nitro plane after flights). I think that covers 90% of what you need.