Just a couple of weeks ago I installed a new skin on the rear seat base of my '01 X5. It has the factory aluminum -- however it is constructed. I used a combination of 3/8" and 1/2" staples and they held just fine.
"For every complex problem there is an answer that is clear, simple, and wrong." (Henry Louis Mencken)
2005 X2, Viper Red, MCX, Acme 1285, PPass, rear 750 sacs, KGB, IBS
(previous) 2001 X5, 1991 TriStar 190