Thats quite strange, I am on the north edge of a hurricane, and have no such cover with a 3lnb, and the only rain fade I got was 10 minutes before it started raining, once it began raining, the rain fade went away.
After reading your post, I sprayed water with a hose on my dish(lots of water, lots lots), with no cover, and observed the signals for 10 minutes, and the signals never dropped at all, no matter where the hose was sprayed, so how would a cover help?, adding a cover would do nothing, in fact it would probably reduce the signal if anything.
I am not saying your making it up, but it may be a placebo effect, there is just no empirical evidence that proves that the act of falling rain on a dish affects signal, in fact the empirical data gathered from the above mentioned experiment proves the opposite.
Here is a text on rain fade and what it is caused by (and none of them are from rain on the dish/lnb itself, the only documentation you will find that states otherwise, are the scam artists who sell the 'anti-rainfade' spray or covers), the only solution is a better dish, period, saying otherwise is simply denying physics (unless the laws of physics somehow change for you), as a droplet on the face of your dish does not have enough density (even if the dish is covered in droplets, its still to thin) to be a real factor.
http://www.solidsignal.com/satellite/rain_fade_about.asp
Quoted from above:
"Rain rate is the most common factor used to determine rain fade. Rain fade seems to correlate very closely with the volume of raindrops (expressed in cubic wavelengths) along the path of propagation. This is opposed to the common misconception that the degree of attenuation is proportional to the quantity or individual size of the raindrops falling near the receive site."
Me: So the few thousand raindrops collected on a dish cannot be the cause of significant (if any) rain fade since the dish does not have enough area to contain the volume of raindrops required to cause propigation problems.
Covers, by the way, do have their uses, as the poster below states to keep the lnb/dish in good shape, and they can look nice too, but covers reducing rainfade? No.