You can use this to haul all kinds of things, you can strap a box or tote to it and put things inside, you can even stop along the road a throw fresh roadkill on this baby (not legal in some states) and save good food going to waste!
This is a good all-rounder and it cost me part of $6.
I used a 1/4 piece of plywood, other thicknesses would work too, but this is really light. I made this thing about 20" long and about 16" wide, but experiment with whatever size you think would work for you. I used a jigsaw to round the corners, but if you don't have one then just cut them angled, sharp 90 degree corners frickin' suck!
Then place it on your bike rack, with it centered over the middle line of the rack, and drill small (1/8 inch or so) holes just to the inside and outside of the rack in 4 or more places and thread wire through and twist it around the frame to secure it. I used what I had available but if you have something heavier, like baling wire or even a wire coat hanger, that would be much more bomber.
Drill larger holes, I used about a 1/4" drill bit, or maybe a bit bigger, something a bungee cord can get through easily, every few inches around the edge of your rack, maybe a 1/4" in from the edge.
Test ride your new rack, strap some things on it, ride it around town. How does it work? Is it what you wanted or is it too big? (cut it smaller) or too small (shit! start over!) too wide or too thin?
When you get it just how you want it sand the edges down real nice with at least a couple grades of sandpaper, get it beautifully smooth and then apply some oil (olive, coconut, linseed, mineral, God, anything really) or verathane or equivalent.
Enjoy your handiwork!