That's a great idea but how could you make a cover that would fit correctly for all cars?
I know in my car I have a very tiny amount of space on the top of the unit and a huge gaping hole beneath it. Others have even larger holes and others have holes so small they can barely fit the Z1. And of course we all have different angles versus our dash boards or center consoles.
I'm not saying it's a bad idea though; in fact it's a pretty good one. If you could design something that is easily trimmable to the correct size for multiple systems you'd have your own gold mine.
That being said plastics molding equipment is not cheap by any stretch of the imagination so prototyping something will be expensive. You could get your own prototyping laser "printer" like a (Link -> )
VersaLaser system ( <- Link) but you're still talking around $8,000 USD.
....
It was just pointed out to me that you're probably talking about a Dremel and a chunk of ABS type of McGillicutty and that I'm over thinking it. ;) In that case you just need a Dremel and a cutting guide. Don't EVER think you can accurately guide a Dremel; they're like angry, drunken women... loud, difficult to handle, and you'll usually end up going in the wrong hole.