You will find that designing the door for a C&C ferret cage will be the most difficult part. I built a C&C cage for my ferrets and it turned out great except for the fact that I could not find any clips that were appropriate for keeping the door closed tight. I did not have a top-open door, but rather one that opened out from the front of the cage.
I tried all kinds of clips but they always managed to squeeze out through the 1" gap between the door and the frame. You must find a way to keep the door flush to the cage, or they will escape. I tried using cable ties on both sides temporarily, but that was just stupid because I had to cut them every time I let the boys out to play. And then when I put the ferrets back to bed, I had to put new cable ties on. It was just a temporary solution until I was able to purchase a new cage for them. So my best advice is to experiment with several door closures before going forward with the build. You might find that buying a cage will be better in the end, as I did.