>>69925082If you want to use a proxy card in your deck, you need to ensure it looks and feels like a real card so when you shuffle, it's not distinguishable from the rest of your deck. Otherwise, you'd need to have the real card in your deck and swap it out for the proxy once it enters a public-information game zone.
Since a Commander rarely, if ever, gets shuffled into a deck, if you're feeling cheap and/or lazy you can just use the proxy without worrying about all that.