best fastener for double skin aluminium onto a steel tube

[David.H]

There are stud versions of Rivnuts called ....Rivstuds ! 0r there used to be when we sold them! Otherwise you could use say - M6x15 into the tube ( or put an M6 bolt into a Rivnut in the far side of the tube, or even a bolt across the tube with a half nut to hold it & support the bonnet) and then retain the bonnet with e.g. a wing nut or a washer with an R pin into a hole in the bolt.
Many ways of skinning this cat! The over centre clip looks the neatest to avoid catching things (skin) on if you can fit it on.....