So I use a plastic tool box for my field box.
I fly mostly 35 size stuff now but will someday expand to have more 049's 15' 45's and the like.
I keep in the box
Handle - two sets
Lines - two sets
Adjustable wrench\
two pairs of pliers
Phillips and flat head screw drivers
glow plugs - about 4 or 5
glow driver
safety thong for handle
props - about 4 or 5 - 10-6 and 9-6
Allen drivers for 4-40 and 2-56
Rubber bands for tanks and landing gear, etc
Stooge line
Fuel syringe plus a fuel bulb (backup)
plastic bag full of 4-40 -bolts, nuts, wheel collars, screws - 2-56
razor blade
Fuel line - about a foot
I carry spares because the field I fly at is 35 to 45 minutes away.
It can ruin a day when you have something simple fail (like a landing gear clip)
and you have to pack it up because you can't find a replacement