I used a medium weight cotton canvas, it's about $11 per yard and I used less than half a yard.
A zipper, the one I used was water resistant and 30" long.
2" wide Velcro, about 18" of it.
White micro-crystalline wax similar to this, purchased at local art supply store. Beeswax or Paraffin would also probably work fine.
Sewing tools(sewing machine, scissors, etc.)
Heat gun, for melting the wax. $15 at Harbor Freight.
Disposable bristle brush, for applying wax.
A heat resistant bowl you won't mind ruining, for melting the wax.
A putty knife or butter knife, for working with the wax.
Step 1: Make a Pattern
Then mark the angle of the front and back, make sure you mark which way is front.
You should also mark where any cable fixtures are so you don't put the velcro straps there.
Step 2: Cut Fabric
Measure the circumference of the piece you cut out, this is how long you want the center strip to be but cut it a bit longer.
I used a 2'' wide ruler to lay out the center strip but you can have more room by making it wider. Just don't make it too wide or your knees will rub on it!
Step 3: The Zipper
Cut the side piece where you want the zipper, all the way down the length.
Fold over the cut edges, I used some double stick tape to hold it in place until I could sew it.
Position the pieces on the zipper so the whole thing ends up being the same size it was before you cut and folded it. You can pin it there and sew along both edges of the zipper.
Next trim off the excess of the zipper and sew a small piece of doubled over fabric over the front end of the zipper, this makes a little pocket for the slider to go into and makes it more waterproof.
Step 4: Velcro
Sew or pin them in place so they are pointing to the middle. Do the other side piece so the straps are in the same locations.
Make sure when you do the other side that the velcro is facing the opposite of the first piece. For example, if on the first piece the loops are facing the ceiling then on the second piece the hooks should face the floor.
Step 5: Center Strip
Start the strip on the bottom so the seam ends up there, it will be easier to make the ends come together if you position it in the middle of a straight section. Leave a few inches unsewn in front of where you start so you have room to make the end joint.
Sew around the edge remembering to make your stitch 1/2'' from the edge. Stop a few inches short of the joint.
Fold the two ends up and make a mark where they meet, this is where you'll stitch them together.
Fold the joint over and complete the rest of the stitching around the edge.
Now sew the other side on the same way.
When you're done, the bag will be inside out, unzip it and turn it inside in!
Step 6: Apply the Wax
Use the heat gun on high setting to melt the wax in the bowl, something to stir it with is helpful but don't use the brush or the heat gun will burn the bristles off!
When the wax is liquid and clear, brush it on the canvas being careful not to get any on the velcro or zipper. Re-heat the wax if it cools and becomes opaque.
You don't need much, if it gets too thick in one spot you can scrape some off with a putty knife or butter knife.
Now the fun part! Use the heat gun on low and melt the wax into the canvas. Be careful not to linger in one place too long, keep the heat moving. Use your putty knife to shield the velcro from the heat because it can melt very easily. Also, don't melt the zipper teeth which are probably plastic.
Woohoo, it's done! Slap that on your bike and ride around in style!