Probably blind rivots as they are waterproof- ish and easy to do at the factory. They often use quite big rivots to spread the load a bit . In alloy you may be able to pull them with a domestic size pop rivot gun but the larger ones you need a better quality rivot gun( get away with a few but small guns will break) or you especially with the large stainless ones.
Jump on one of teh Yak sites a ssomeone local will have a decent gun to borrow.
Screws - not enogh polly to bite into at a lot of spots and need backing and watch out for sharp points They do use very coarse thread large guage ones. Again the yak sites will tell you what is best.