I'm having good luck with the Retrax Pro! Works best on long box since you do lose some space to the roll-up canister.
It's a solid cover, locks at any point along the track. You can roll it pack partway to cover the box in front of the hitch when pulling the 5er.
Thats what I like the most about mine as well. I can throw the firewood in front of the hitch, hitch up the fiver and move the cover back to cover the firewood if the weather looks iffy. Also since its retractable the wife or kids have no problems using it.