How to create a structure in which certain entities will appear by themselves?

To the best of my knowledge it's not possible to do this with MCreator. I know you said no spawners, but you might still consider using a special block or entity.

For my structures, I'll often have a special block that's programmed to spawn entities in a radius when randomly ticked- and then players have to break the block to disable reinforcements. You could also make an invisible, persistant entity with no hitbox, that does the same thing when loaded, if you want it to be subtler. For a surface structure, the procedure's pretty much just setting a random x/z offset, and then spawning a mob at world height, with the random offset applied.