OP I don't have much wisdom to share about how the design will sound BUT I do have a question that might save you some cutting mistakes...
If you go w/ your design that you drew in paint, unless I am reading it wrong, the measurements don't add up...
you say your port will be 18.25" high, then it "looks" like you will have a piece of 3/4" MDF, then 4" on the bottom. Finally, you say that the overall height will be 24".... see where I am going? 18.25 + .75 + 4 = 23"
Something isn't going to work how you are planning