Yes, agreed. Thanks for that Reinhardt "Rain Shield" ~5cm overhang figure. I ball-parked the needed overhang at around 7cm or so as likely to be about right. Even with that amount of overhang I would want to make sure the windscreen underneath was treated hydrophobicly to reject moisture uptake as much as possible. I tried the NeverWet stuff on a pair of BAS a few years ago and it worked well with regards to indirect water/moisture application, but was not sufficiently hydrophobic enough on its own to resist water intrusion from direct physical droplet impact to the windscreen. The fabric cover of the Movo's is probably superior to the foam exterior of the BAS somewhat in that regard, but would still need the top cover and impact diffuser.
The umbrella fabric to be glued to the underside is intended to serve as water barrier same as the polyethylene wrap you suggest. Either should work in that role I'd think. Advantage of the umbrella fabric as I see it is that it is thin, pretty tough, and black. Should probably do some tests of audible transparency, but would certainly be less reflective/blocking than the Reinhardt plastic shingle. The fabric should extend to the outermost edge of the overhangs, and form an outward lip along the sides to the outside edge of the foam to form a drip edge.
Edit- Note that the pond pump intake filter stuff I used for this initial prototype is probably not long enough. I ideally need to find the same stuff in longer lengths. About half of the length of both overhangs in the photos above are separate pieces of the same foam material pinned on (pins are visible in the photos). However if I can't source that I can probably glue two sections together with a nearly invisible seam in the middle, or better, maybe 3/4 of the way back but before the overhang angle cut, which would be well behind the mic diaphragm.