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.
2005 X2, Viper Red, MCX, OJ TZ-X5 prop, PPass, rear 750 sacs, KBS, IBS, Bennett Wake Plate, custom wake shaper
(previous) 2001 X5, 1991 TriStar 190