Okay this step is by far the most important, but leaving a little leeway is a good idea too. I can't remember what exactly we measured and the truck isn't with me so all lengths are my approximations.
The four measurements are:
The width of the bed, which was around 4-5 feet
The length of the wheel well protrusions, this the longer forwards to back one which is around a foot and a half. (I'm referring to this board as the supports from now on)
The width the wheel wells come out, this being the shorter one less than a foot
And lastly the angle of the wheel wells. We came up with 45*. Angling it isn't necessary as it is for aesthetic purposes, but we found out that because of this the bags pushed it upwards. To fix this we simply placed a bag on top of it once finished.
TIP: For the scalar lengths we suggest leaving around a half inch off what you measure so it is tight, but this way you could take it out easily in the off season.
Step 1: Chopped!
Next step in taking shape is to cut the width and the support beams. When marking the angle on the board it is important to remember that the longer side of the "trapezoid" is what you measured for the support's length.
Step 2: Screw This!
No literally... (My sincerest apologies)
Laying out all the boards on the floor is likely the easiest way to line them up.
Mark in on the main boards the sideways width of the wheel wells, this will be the edge of the board.
I suggest two things when drilling these: to predrill as end grain can crack easily, and a second set of hands to hold the board while you drill and screw it in. Plain and simple really, the zoomed in photo does a good job of describing it.
Step 3: Crossing Our Fingers It's Not Too Wide...
Phew it fit! Lastly fill it up with your bags of sand. For a quarter-ton truck like this the gentleman at our local hardware store suggested 200-250 pounds, but larger trucks need more weight obviously (ie ask google what your vehicle would need)
All in all this was a quick and easy project that took at max 2 hours including store time and came out better than expected.