Nick, what you want is some lead fashioned into a cone shape - about 2 and a half inches thick should do. Although you will need to 'brace' the mounting points so that they are 4 inches thick.
Ideally you will want this covering as low as the front cylinder head right round over the top of the clocks to a height of approximately 2 foot.
HTH
TP
[Edited on 26-2-2005 by tp-996]