Online you can find models costing around $60. But this customization will cost you nothing and will only take a few minutes.
Step 1: Find the Tools
This step is really easy, and relatively primitive. All you have to do is find something heavy to slide into the flap on the back of your mower. Anything will work: a pipe, BB's in tubes, a Maglite, a crow bar, ratchet set, PVC pipe full of sand or water... You get the picture. I went with the 'ol crowbar and a set of sockets.
Step 2: Insert your Items
Place your weight in the black flap on the back of your mower, the tighter the fit the better. I had no problem with it sliding around; if you do I would suggest duct tape to hold it in place.
Step 3: Mow!
With the new weight you will be able to make nifty patterns or just enhance the bread and butter straight rows. When turning the mower you have to make a little bit wider turns to prevent backing over the flap. Remember, to get the striping you must go back and forth, each pass should be opposite the other.
It will probably take you longer to read this Instructable than it will to modify your mower. This is something easy to do once a month or every time you mow. You will have better results the taller the grass is. Mowing usually isn't enjoyable, but hopefully this will enhance your chores. Have fun!