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Passive dew shields do a remarkably good job, but I have had them "fail" on those humid nights after say, 2 hours. Generally, the optics are more vulnerable to dewing up when pointed at the zenith, as the cross section of glass exposed is larger compared to pointing at say, 30 degrees.
I used closed cell foam camping mat to make mine. Some people use those thin plastic sheets used to bind computer paper. I think the recommended formula is to make them at least 1½ times longer than the tube diameter.
Basically, long is good provided they do not cut into the field of view, or become too heavy or act as a wind sail.
Cheers
Dennis
PS – closed cell or chemically blown foam is tough, does not absorb moisture and is easier to cut than the cheaper, sponge like foam.
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