best fastener for double skin aluminium onto a steel tube

[Stuart Giles]

You could drill a hole though the bonnet and through both sides of the tube and use Pip Pins to hold the bonnet. you'd probably want to make escutcheons from stainless steel penny washers to stop the pin galling the hole in the (presumably) ally bonnet.

Here's one style of Pip Pin - As well as dozens of different diameters, grip lengths, there are numreous different head end styles of these; here's one type: https://www.coburg.co.uk/en/catalog/quic...dle/pp1210