Both the sentences are actually not correct. It would be better to say: "When you walk under a ladder, a can of paint may/might/could/can fall on you." The difference between "have" and "make" in the sentences you were trying to say is that "have" is about experiencing [a can falling on you] and "make" is about causing [the fall of the can of paint]. If you want to use either of these words associated with walking under a ladder, you will need to create more context.

To "make" someone do something implies a certain forcefulness. You compel the person to do something they would not otherwise do. To "have" someone do something is more neutral, although it implies that you are in a dominant position that automatically gives you the right to give orders and tell others what to do. That "dominant" position could certainly be due to the fact that you are paying someone to do something for you. To "get" someone to do something implies effort, convincing, patience, etc., and therefore a relation between equals. You are not in a position to simply tell the person what he must do. Instead, he has to agree to do it on his own terms. To "get" something done & to "have" something done are essentially synonymous in the context where they imply (but do not specify) the person who will do the work. The version with "have" sounds slightly more elegant (or snobby, depending on your perspective).