You can represent conjured cards using objects from outside the game so long as the game state remains clear and understandable to all players. You might need to get creative. For example, while a conjured card is in a public zone, you can represent it by writing on a piece of paper or using some kind of token. But if a conjured card is put into a hidden zone, you must add an object to that zone which, while face down, is indistinguishable from other cards in your deck.