You can print out a picture of the bottle and cut it out and set it on your rectangle cake and use it as a guide to carve the cake. Even if it's longer than 11 inches, just print out the top and the bottom and then tape them together. Save it, because it will be useful in placing the labels on accurately. You would want to use a firm cake.
Even though the packing material looks like fettuccine in the photo, I'm sure it didn't in person and that the person the cake was made for wanted it to match the peach ;) That's a really good tip about using a bottle as a mold, but the problem with using a real Ciroc bottle as a mold is that Ciroc is expensive and the bottle wouldn't be returnable. However, it can be really helpful to buy a bottle just to have in front of you in person to cut out the shape (and then return it it).