best fastener for double skin aluminium onto a steel tube

[spannerman]

I use 4mm stainless rivnuts on mine and
Use 4mm mushroom head stainless screws
But you must put on copperslip on them
Or they will seize from a few feet away the
Look like rivets

[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

[Colin Cooke]

How about just fitting spring clips onto the underside of the bonnet? They will just snap onto the tubes.

Good luck,
Colin

[JonE]

Colin - the tubes are under the main bodywork...
Stuart's pip pins are lovely, but it would mean the 'pip' was just retained by the inner edge of the tube, which is probably not very thick steel. Perhaps with an insert...
Duncan's route - yes, I'd agree, too modern for me too. (And no space underneath to allow nut fixing on both sides of the tube).

I've bought some tiny M4 wellnuts and knurled knobs to try out in a conventional manner and see if they are up to the job. I will be interested to see if the rubber confers some safety against them unfastening in use. I reckon this will be just a first experiment, as unscrewing 4 knobs with a 12mm top (with a slight offstand) is still probably going to be a ballache! The bonnet probably only weighs 500g or so.

[David.H]

Colin's suggestion is really simple, you might need a handle or raise a lip so you can get fingers under it......could be the deicer device if you put one at the rear of the bonnet! 
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