I confess I had to look up what a spire nut is. Thanks.
No worries! When the heat shield on my Mk1 Octi decided to start letting go, my mate said "you need a spire nut on that". Like you, I'd never heard of them. Every day is a school day.
These Rivnuts are useful as well ( like a pop-rivet with an internal thread ) when you want a one-sided-thread in thinner material (when no access to other side of part ). You can buy all sorts of fancy insertion tools but do not need them
all you need is a bolt and a washer
Drill the correct size hole, push the rivnut through, put washer onto bolt ( a bit of lube on thread** ), screw the bolt into rivnut, continue screwing bolt in until the rivnnut has collapsed ( the body of rivnut above the thread is designed to collapse and form a collar, just like a pop-rivet ) unscrew the bolt - there you have a proper nut ready to go (** it is better to put a bit of oil on the bolt as that will reduce the thread friction and reduce the tendency of the nut to try to rotate in the hole when it is being 'squashed'). The correct size hole is important, the splines on the rivnut dig in to material to stop rotation.
Material they can be used in depends on the thread size, from about 0.6mm for M4 size to about 5 or 6 mm for M10 size. These would be ideal for fitting a CAT-Guard to MK1 Jazz, they are available as 'blind' ( hole does not go straight through ).
https://www.amazon.co.uk/gp/product/B08JC7GGNQ/