Hi MrX,

Excuse the ignorance, but I hadn't appreciated:
(a) what a heat shield is, and 
(b) what problem was being solved!

So I have found out that a heat shield protects parts of the underside of a car from the heat of the exhaust pipe(s). It's a formed sheet of metal that is bolted to the underside of the car.

The problem was that the mounting holes in the metal sheet had corroded and become larger than the bolts and washers holding it on, leading to nasty rattling. The metal tin lids with holes drilled in them act as giant washers, holding the shield on again and preventing rattling!

I realise that this was a great hack - now I understand it!