your 29.75 measurement is off, you forgot to subtract the 1.5" on the other side of the port as well. also, if you add together all of the dimensions inside the box, you will come up with 38.75" wide. the 9.5" is also off.
then just draw whatever you have there in, and if it dosnt come out right, you will be able to see why. basically its like using a ruler and drawing 'a scale model' of the box on paper first.. and making sure it all fits.
id look into some form of CAD //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif