Your question has a lot of room for subjectivety.

As far as what needs to be in a flight box aside from the obvious, fuel, fuel pump, glow starter, engine starter, etc. I also carry a glow plug wrench, spare glow plugs, push rod clevis, control horn, small screwdrivers for adjusting needle valves, larger screw drivers, wrenches, and allen wrenches for removing spinners,props, wing bolts, landing gear, etc. I also keep needle nose pliers and tweezers for picking up dropped screws and fishing fuel tubing thru a cowl, a tachometer, and some paper towels.

As far as what makes a good flight box: When I was doing most of my flying at the club field which had starting tables, my flight box did not have to pull duty as a restraint for my plane, so I tried using a large tool box. Worked fine at the club field, but now I do a lot more flying at home and other places that don't have starting tables, so I went back to using a flight box that has cradles for holding the plane while I start it.