How do I make a block that keeps it's inventory when pushed by a piston? And how do I code the command block creator?

I've made a custom crafting table that will help the player in creating the Command Block in survival mode(I already added a way to use it.) but I'm having trouble making it so that the command blocks keep the items they have in their inventory when they're pushed by a piston, or something similar. I would also like some help making it so that the custom crafting tables sense what is in the inventories of nearby custom crafting tables. I want to make it so that when 4 of them are facing each other, and when they all have the correct assortment of items in each table, it will create a block in the middle(The Command Block).

I want to make it look like the machine in MCSM(Minecraft: Story Mode) in the scene where Ellegaard is trying to make the command block.

-A specific assortment of items in each crafting table (a specific 9 items in one, a different set of 9 items in another)

-Make each custom crafting table sense when all 4 have the correct items, then make the command block appear in the middle(I have a custom command block that will be pick-upable).