unobtainable dummy items

Since you can't change the item components for recipe inputs, i want to make my few custom recipes i have survival proof (so you can't get them by using the non custom item). Until now i used poisonous potatoes, but you can obtain them in survival (i know its difficult but still, they are technically farmable) and i want a few others (probably like 3-5) that i can use as different custom items.
I thought about using blocks like command blocks and test/structure/jigsaw blocks, since you can't place them in survival even when you are operator and for my use case i don't have any other operators than me.
My question is am i missing something important or can i safely use them for custom recipes? Or do you know any better option than to use them?