Specifically, what is the reason you want to use a software enclosure design program?
Let me start by saying, even if I was to give you software, what are you expecting to get out of it?
I am a firm believer that these applications are created to give you an estimation. That's it. Most serious enclosure designers/builders wont touch these apps, after all, most of them are innacurate, and there is nothing better than busting out a pencil and some paper and doing some good old MATH!
If you are trying to calculate port volume, dimensions, etc. There are plenty of knowledeable members ready and willing to help you out.
Don't rely solely on software. God gave us a brain for a reason, to use it!!