I'm gonna go out on a limb here and guess this has been done before.
In my tray (I have a multitude of other tool boxes in the van, but this tray covers most eventualities) I have....
Screwdrivers - flat and phillips, two 8" wide jaw bhaco adjustable spanners, two sets of Rothy grips, electrical side cutters, long nose pliers, cable strippers, a claw hammer, a stanely knife, a tap measure, ptfe, jointing compound, a remover tool for polyplumb joints, pipe slices 15/22, a magnetic pick up tool, a pencil, a rad wrench, a neon tester, a boat level, a 15/22mm speedfit stop end, formers for the bender, a pump tool, a set of allen keys (metric only) and a wratcheting screwdriver with a tin of various heads including a small set of hex sockets.
Probably missed something out, but that is my bread and butter tray. I will always need those tools at the least. Things like U guage, smoke pellets, soldering gear etc might all stay in the van on some days, but that tray gets taken out on every single job.