This, I must add, is pretty old stuff.
I did a City and Guilds in Motor Vehicle restoration and I used a very much simpler set of clips.
I'd made a set of fasteners on the lathe but then discovered that Frosts, an old established body work shop sold them in packs of 10.
This is nearly 40 years ago and mine were made of 1mm steel for insertion between high strength low alloy steel which had been airhacksawed from Nissan panels and which we had to Mig up again.
Basically, 'mine' were scraps of flat steel radiator sheet with two holes and taper pins which nipped up the panels. If bits were lost in an enclosed section, so what, they were cheap enough.
Even the Nissans were 'free', we got them as rejects from the local factory and hacked and chewed them-- and handed them back simply to satisfy the tax man and his accounting.
It was, and is no great thing.
Regards
Norman