Is it really worth the effort for one 250 mile trip?
How much time & work are you willing to invest to save less than seven dollars? (I'm assuming saving money is your goal.)
The math:
- You'll use 17.86 gallons @ 14 mpg (250 miles / 14 miles per gallon)
- Let's say you improve your fuel economy a reasonable 10% with a quick & dirty aero shell.
- That means you'll save 1.786 gallons, or $6.79 at $3.80/gallon.
You could probably save more than that just by choosing an alternative route that lets you drive slower without disrupting traffic.
I think Sven's advice on the half-tonneau is a good compromise on return on investment for this case. Could probably slap something together out of used building materials in 15 minutes. Throw on a partial grille block, fold back the passenger mirror, etc., etc., etc.
(PS - I also read recently that the bed needs to be enclosed under that rear half-tonneau for best effect, meaning: there needs to be a vertical piece at the forward end between the bed and the tonneau.)