The lift on my 2000 GMC 1500 was enough to cause it to want to get "Loose" in icy windy conditions. It was really kinda tough to drive at highway speeds in the snow, particularly when passing a big rig. When the aerocap was on it never felt like skipping out. It is hard to describe the effect you'll have day to day, but I noticed my truck was quieter and smoother, and it just felt like it wanted to go straight down the road more.
Just get a 12 foot 2x4 and rip a 1x2 off it, put the back of your "Arc Stick" above your tail gate and add weight to the front and back, or tie it down somehow, till it droops to about 6" above your tailgate. Just be sure the transition from cab to cap is smooth, there should not be a drastic angle change anywhere.
About the sides, if you keep the aero cap a bit higher rather than try and follow the curve, I reason that you'll create less of a "Low" pressure zone on top of the cap, this will help reduce the amount of air that feels the need to go up along the sides of the cap to "fill the void". I noticed some pretty seriously upward angled water streaks on my cap even with a side taper and radiused edges. My cap was much too steep.
Do radius the edges! If you make square edges, it puts high & low pressure areas in very close proximity and will create Cd eating vortex generators on each side of your cap.
