That's excellent, a job well done, it really is. It doesn't have to look fancy, if it does the job you wanted then it's perfect. The cats seem to love it, and Im sure it'll give them hours of entertainment being able to go in and out as they choose, they'll be able to taste a bit of fresh air and freedom, its' brilliant, and you should be proud of yourself for putting in the time effort and care trying to give your cats a better life. You are thinking of them and I'm sure they appreciate it ten fold.
Well done on being a great cat lover/owner.