In past years I lashed plastic cat litter containers filled with sand in place, but these would slide, and sometimes break open. This year I still used the cat litter containers, but built a wooden base to keep the containers stationary. It worked so well, I thought it was worthy of becoming my first Instructable.
Step 1: Get Stuff
2x4s (at least 20 feet worth)
three plastic cat litter containers with tight-fitting lids
enough small rocks (1" to 1-1/2") to fill the containers
Surform or other sander
Step 2: Drill, Baby, Drill
Find a free source of small rocks, preferably without stealing from your neighbors. Select appropriate rocks, rinse and allow to dry. Fill containers so that a) there is a minimum of space to allow shifting of rocks and b) you can close the lid tightly.
Step 3: Measure Truck Bed
Measure the sides of one of the containers. Now, face the truck bed. Measure from inner sidewall to inner sidewall at the widest point. If it looks like your project is going to infringe upon the wheel wells (the Sonoma has a relatively short bed, so that was my case), measure width between wheel wells at their narrowest point.
Step 4: Build Your Base
Drill pilot holes suitable to nails, and nail your wood pieces together.