I made dew shields for my 8" and 10" newts and also my 180mm mak with 2 or 3mm foam rubber similar to wetsuit material that I purchased from a store called Clark Rubber here in Perth.
You just measure the outside diameter of your scope at the end for the length you require ... allowing for an overlap of an inch or two ... I made my shields about 30cm wide ... cut the foam rubber to size and then attach self adhesive velcro ( buy it from fabric shop ) to both ends of the rubber. From memory the rubber cost me about $27 dollars for the roll and the self adhesive velcro was very cheap ( a few dollars again from fabric store ).
They work very well and I still have lots of materials left over if I should need to make any more dew shields in the future.
p.s. I just read your latest post and see you would rather have commercially made shields ... now worries !