I use 2-80 lbs bags of sand encased in burlap and plastic (purchased at Lowe's) for extra weight in the rear of the bed. I also add a 2x6 board across the bed to brace the bags close to the tailgate area to keep them from sliding around on the slippery bedliner. It gives my truck close to a 50/50 weight distribution over front and rear axles and really helps with how the truck handles, improving it quite a bit.
As far as a cover for the rear bed, I am thinking about using a soft topper available at Softtopper.com It works like a convertible top you might see on a car. Since I live alone, its advantage is one person can install it and take it off. It can also be folded down and rolled up to create an open bed when you want one for hauling large and bulky items. Disadvantages: can't take your truck thru a carwash with it on. I am still pondering that purchase. I can't recall how it compares price wise to flip up tonneau covers. You might want to compare prices.
'08 Frontier, 4 cylinder, A/C, cruise control, manual 5 spd, radio/cd, with CREE LED back up lights from JLC
Great truck, purchase a 2012 pro 4x cc this month myself. I installed a bakflip fibermax yesterday. It looks great. Only took about 20 minutes to install with no drilling. Purchased for 659.00 delivered. I would purchase it again.
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