It rains here in the Pacific NW. I've just made another rain cover for my pack. The first one I made from urethane coated ripstop (80 gm). This one I made with siliconized nylon, total weight 38 gm.
Above: the raincover on the backpack. Note how the cover doesn't wrap around the bottom of the pack. 
Above, the bottom of the pack. Elastic runs across.
Above, a top corner of the pack cover, showing a decent fit.
I have rain tested the cover. EDIT: I use a GoLite umbrella in the rain, so rain doesn't drip off the top onto my back. If I didn't use an umbrella, I might work on a hat add-on that would cover the top of the pack
Below, quick and dirty instructions.
Cost: one yard of silicone coated nylon (most likely 54-60” wide), about $10-15.
small elastic cord $2 (?)
For my backpacks I cut the nylon to 42” x 34” FYI, My backpacks: REI Flash 62, Ray Way 3000 cube backpack.
Construction:
1. Make a rolled hem for the bottom, which is one of the 34” sides.
2. Decide which is to be the right side or the finished side, if it matters to you.
3. To make the corners, go to the other end & fold one corner matching the top edge to the side edge. Make sure right sides are together. The angle of the edges to the fold will be 45 degrees. Pin the edges together at 8” and 12” from the corner. Make a mark on the edge 10” from the corner. Make another mark on the fold 14 ⅛” from the corner. Connect the two marks with a line. The line should measure about 10”. Sew along the line. Cut the small folded triangle of fabric about 1" from the stitching and discard or save for a 5 gm wallet or something else.
4. Repeat the last step for the other top corner.
5. Now sew a ¼” hem along the sides and the top, whose edges are continuous.
6. Make a sleeve for the elastic. The sleeve runs from near the bottom of one side, over the top to near the bottom of the other side. The elastic will go all the way around. The bottom of the rain cover will hang freely, so that any water that drips down keeps going to the ground rather than following the bottom to the back of your legs. For my backpacks, I began sewing the ¾” sleeve 6” from the bottom corner, continued up to the top and finished 6” from the bottom on the other side.
7. Attach 7 feet of elastic cord to a safety pin or short pencil (using a rubber band) and temporarily tie the other end to something that won’t slip into the sleeve. Pull the elastic cord through the sleeve and tie to the other end.
8. Add a cordlock to the elastic if you like (I didn't). You’re done.
