not exactly water/dirt proof but I'm currently using a set of these. Smittybilt
katch-all covers. Lots of adjustability to get a tight fit. I use the pockets on the back for lots of stuff so I like them for that. I looked at the G.E.A.R covers from Smittybilt that are similar and more durable, but the reviews weren't great and they were double the cost so I'm trying these out. Had them for two months, no complaints yet.
These are the G.E.A.R covers. The fabric is really durable and I really like the Molle attachments on the back so you can configure the packs anyway you want. My only beef was the cost and that the fitment is a little loose. If you adjust the straps to get a tight fit it puts too much stress on some of the seams and they can split over time.
these come in Olive Drab, black, digital camo