Custom 'Simple' Loot Boxes using Shulkers
('simple' cuz you need to understand and customize loot tables a bit, and utilize command blocks)

Just a little asset I made for a randomly generated roguelike map I am making.

Uses 1HP coloured shulkers with no AI, each with a specific loot table.
Hit them and get loot.
Put the commands in an RNG machine or something.

/give-able using spawn eggs, or /summon-able at a specific location (requires a command block, string too long for chat)

Separate loot tables for each loot box (8 total, 7 colour and 1 for test). Just edit them to customize your loot.
...\datapacks\LOOT BOXES\data\lootbox\loot_tables\entities

For copy/paste commands, see README's.
100% when I make the code look nicer and all that gravy stuff.

Adding a custom villager trader that sells the spawn eggs (RNG shop) - comment or message if you want it, it's pretty simple.