found it very easy to replace , cheapest Universal waveguide cover I found was :
http://shop.ukwhitegoods.co.uk/Microwave%20Spare%20Parts/Universal%20Microwave%20Items/RL75101%20Universal%20Microwave%20Waveguide%20Cover.html
just cut to size using your old waveguide as a template .
William Anker
June 2010