buy tech screws. get the ones that come somplete with rubber washers. factory cladding does not rust anyway, the rubber washer allows movement and seals the hole at the same time.
various sources sell tech screws and prices vary. screw fix arent the best on price but lets you get hold of them quickly. http://www.screwfix.com
you will only need them to be about an inch long.
use only "full thread" version. i.e. the thread runs length of screw
you WILL Need a tech bit (see picture below)
dont buy star screw head versions, get the Hex-head, much easier to work with.
if your using a panatrim type frame, you may not get a tech screw in the space unless your using techs without the rubber washer.[/u]