Based on a recommendation I was given on this forum I was able to purchase exactly what I needed; thus successfully ending a two year search to find a suitable replacement instrument cover. I found a 5-inch diameter circular cover manufactured by SSI of Hollywood MD. In case anybody else is looking for the same item their web site is www.ssicustomplastics.com
(as of January 2007). In addition to the instrument cover they have numerous other marine products. I paid $6.29 for each cover (I bought two) and it cost me $9 dollars and change to get it shipped out to the west coast. I had to call the phone number to place my order as I could not find a way to order anything directly off their web page. Otherwise the purchase was simple.