I cannot recommend something special. There are no recipes for best tool boxes. When I was learning my OWD, my instructor emphasized on having spare Ist and IInd stage orings, keys, din adaptor, mask and fins strap. Many people will ad some silicone grease, manometer hose connector swivel etc. After my experiences I know I need spare manometer hose or spare manometer, anti fog lubricant, scissors, spare batteries, even spare gopro and spare underwater camera, I also need many wrenches, but also when we consider volcanized rubber drysuit of Viking brand that I have, I need also many glues and pieces of rubber to fix it. The gas-proof zip also need special wax. Hard to mention all necessary things. It comes to us with time. Experience is something hard to share. And I also decided that service toolbox needs to have function of basic first aid kit. Oxygen and mask is obligatory. And to have special DAN set is completely pointless. Ordinary oxygene tank weighs. And 1 liter can with pressure 14 bar is really everything when we need to inhale some oxygen in emergency.
Note that orings need special tool – metal hook to detach them from the thread on connectors, screws etc. Without it, it is almost impossible to replace them. Or oring just is broken and easy to dispose the used one.