What makes this lens ideal for sleeve protection (aside than the $1459 price tag) is the fact that this lens zooms by retraction rather than by rotation
According to Rob, The fleece by itself is double sided and it already makes it a bit water resistant. But to be safe it was soaked in a water repellant solution called NR.2 protector for 30 minutes and after drying it in the dryer machine, it was sprayed with shoe water repellent spay.
After that, when water is put on the fleece they just roll off.
P.S. If you don’t have camo fleece lying around, you can always cut your pants.
[DIY 100-400L Rain and camo fleece | Rob Gipman]