What if it rains?
I had the 'open-bed-dilema' too. Sandbags, rags, chocks, chains, blankets, hold-downs, tie-downs, tools, and just about anything else worked OK for me.
I got tired of it and splurged a whole $600 on a used fiberglass cap that happens to match my CHEVY TRUCK perfectly. No more problems.
Ask yourself how much all that time it takes to secure your stuff to all the do-dads you had to make or buy is worth to you. Caps are great and for the stuff that won't fit under it the utility trailer works just fine. Especially for my new four-wheeler!
Good ideas thogh guys!