"filling" a named empty shulker box

so basically what i want is:
when I'm holding an empty, brown, named (by anvil, with the name "Test") shulker box in my hand, i want it to be "filled" with a single book and quill, preferably in the dead-middle slot.

i was thinking you'd use something like

/execute as  if items entity  weapon.mainhand brown_shulker_box[<insert nbt stuff>] run item replace entity  weapon.mainhand with brown_shulker_box[<insert nbt stuff>]

but I'd havent a clue in the world as to how to translate "empty" and "with the name 'Test' " into NBT.
can someone help? cause i dont even know if what im trying to do is even possible

info:
26.1.2
PaperMC
Geyser mods

(despite the mods, commmands seem to function propperly)