You should be fine....way back when I was in the boy scouts, we made our own waterproof backpack covers. If I remember it right, we used wax on the seams and then used seam sealer (can be bought in any outdoor section at wal mart or target or any store that sells camping stuff). We also always used seam sealer on any new tent. Seemed to do a pretty darn good job of keeping us dry.
Amazon.com : Gear Aid SilNet Silicone Seam Sealer Tube (1.5 -Ounce) : Camping And Hiking Equipment : Sports & Outdoors