If you can cut an even circle out along with the holes for mounting it plus allowing for which way you plan to run the power cable. All measurements would have to more precise as well as seeing no sharp edges or burrs from this however.
Location of the opening will play a role too. With cases lacking a top opening sometimes heat will get trapped especially around the front since a supply has it's pwn exhaust as well as the case fan seen at the rear of the case itself. Warm air can settle up in around the top around the front mostly.
Is it necessary? Newer style cases now see vented grills replacing simple solid plastic drive bay covers especially on gaming models. Those tend to act like vents for seeing cooler air brought in without a fan placed there.
The other thing of course is simply not making a mess out of any case this is tried due to lack of experience from working with materials. For most simply choosing a case with good air flow throughout the case works easier then making an "ut oh". One case in mind already sees a large top fan placed there by Antec in their 900 model.