Take a look at plastic snap-fit design guidelines. You can probably find it easily enough if you do a search on the internet. The only thing that is really different are the material properties between aluminum and plastic.
Aluminum Association has a design manual which can be purchased from their website. I use it every day and answers all of your aluminum design questions. I found by using this, the mills give fast turn-around to die designs when you spec as per the design manual.
