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, OJ TZ-X5 prop, PPass, rear 750 sacs, KGB, IBS, Bennett Wake Plate, custom wake gate
(previous) 2001 X5, 1991 TriStar 190