What would be the best entity to use as an invisible wall?
Entites --> Technical --> "block_box"
It's there for invisible walls and stuff.
There's one block box in the technical category that ends with _nochar - that will only block entities and such, not the player - while the one above it will only block the player, not entities.
As a personal note, after a bit of testing, it should be noted that invisible walls and entities that collide with the player may cause game-breaking issues (e.g. falling through the floor, getting stuck between an entity and an invisible wall, et cetera). So take care to remove collision for entities that "leak" through invisible walls.