you can get guards all over, a quick google search brought up
http://www.discountedheating.co.uk/shop/acatalog/Square_Terminal_Guards.html
measure flue terminal and pick a guard that ensures when fitted, a minimum 2" clearance from any point of terminal to the guard, ie 2" from face of terminal to inside of guard, 2" from top,sides,bottom etc etc
iain
July 2008