Why not avoid the umbrella completely and build your own "Rain Hat"?
See my comment in this thread: http://taperssection.com/index.php?topic=174011.45
^This is where I'm going next, eliminating the umbrellas entirely.
Thinking of modifying big-ass Sure A81WS windscreens with individual built-in DIY rain hats. I've already moved to using microphones which don't have problems with moisture. But still need to protect from wind and prevent rain saturation of the windscreens and mics.
Plan is to affix rain-hat layers on the upper surface of the individual big Sure windscreens using a touch of spray adhesive to fix a top layer of the super-thin dry-cleaning plastic bag material under an over-layer of 10-PPI coarse PUR material. The thin plastic should be even more sonically transparent, thinner, and even less bulky than the thin layer of polyethylene foam used in the rain-hats in the thread if_then_else linked. I'll bevel cut the edge of the oval shaped PUR filter material to taper into the shape of the windscreen, essentially making the whole screen a bit thicker on top in profile. May need to stitch it in place to the finer A81WS outer foam along the edge at a few points. The PUR filter material will break up droplet impact noise and protect the thin water-tight plastic layer beneath. Water will then run down the plastic and off it's outer edges to the outside of the windscreen. Remainder of the Sure windscreen foam below the upper-surface hat level will be Never-Wet treated to make it hydrophobic, avoiding water saturation. The Never-Wet treatment alone has done a good job protecting the BAS foam screens from mist and moisture saturation, but cannot deal with direct droplet pelting saturating the top.
Sonically, I think this should work nicely- the sonic-veiw along the horizontal plane from each microphone's point of view should be clear without any additional materials interposed in that path. The PUR material is the same as the inner layer of the A81WS, and I know from previous experience that the thin plastic layer can be sonically transparent.
I'm especially motivated to do this because my oddball arrays require a rather large umbrella to cover the four central supercarioid mics, each inside it's own A81WS sometimes along with a coincident bi-directional. Even a large enough umbrella which covers the central mics doesn't come close to covering the 2 spaced omnis which are like 5' or 6' apart. Fortunately the omnis work even submerged, but direct impact rain impact noise and windscreen saturation is still an issue with them, so I may do the same with the smaller windscreens I use on the omnis.
If this works it will be great to have individual wind and rain protection for each microphone position, in a setup I can use outdoors regardless changes in weather, with the most minimal increased visual impact I can imagine.