[1.12.2] Custom Block Chest with Custom Loot

So I've been searching all day long for answers and tutorials, and tried several different things. All in all, nothing has worked for me so far.

My idea is to create a few structures with a treasure chest. I read about how 1.12.2 doesn't really support loot tables properly so I decided to look for another way.
I found a tutorial that first creates a Block Chest (or Chest Block?); a block that acts like a chest with a GUI, like barrels (I made a crate, and the GUI and everything works). Then I followed a tutorial on how to make this block chest contain custom loot in it using procedures. It was this tutorial: https://www.youtube.com/watch?v=h5CYq1lFFj4
I followed everything, double checked all the procedures, used all the tips and it still doesn't work when testing in game.

I also followed a tutorial on how to add a loot table to normal chests using this tutorial: https://www.youtube.com/watch?v=O0pbrlK3VJM&t=212s
But, again, this didn't work or help me figure it out. 

I had a few mods containing structures with custom loot (they did use regular chests though, I suppose it is possible in some way outside of MCreator?) so I know it's something that should be possible... I assume.

Does anybody know how I can do this for 1.12.2? If anybody knows a workaround or a different method, I'd be happy to hear it. 
If there are already discussions with solutions, please let me know. I searched and couldn't find them myself (or they haven't been of help to me). Thanks in advance!