I've been roofing a shed with box-profile steel. TEK stitchers are fine going into 0.7mm panels but don't work well in the few places that need a 0.5mm trim attaching underneath a 0.7mm panel. I can't get to the inside of this section so am thinking of using "spider" pop rivets - the sort that splay out into 4 curled arms when pulled - but they need to be S/S to avoid galvanic corrosion. Any suggestions for where I can get some 4.8mm, or 5mm, stainless "spider" pop rivets? Alternatively, are there any other fastener types I should be looking at, or is there another name for "spider" rivets?