Pickup bed liners protect the bottoms of truck beds. They are usually made from high density plastic that is sprayed on, or a moulded form that fits your particular model. But you can make your own bed liner that's even tougher than plastic and that can absorb more shock by cutting it out of plywood. Most truck beds are already 48 inches wide, and after making cuts for the wheel-wells, a standard sheet of plywood should drop right in.
Shut the tailgate of your truck. Pick up the plywood, centre it and set it on the back of your truck, let the plywood drop into the bed. If it sticks up in the back, use a straightedge to draw a line across the width inside the tailgate.
Turn the plywood perpendicular to the truck bed. Use the jigsaw to cut off the end where you drew the line. Turn the plywood parallel and let it drop into the truck bed.
Reach into the truck bed and make a mark on the plywood centred at the top of your tires as if you were looking through the plywood at the top of the tire.
Pull the plywood out of the truck bed and set it across two sawhorses. Measure the wheel wells inside the pickup bed. Using the centre mark of the wheel wells, use a straight edge to draw the dimension of the wheel wells. Draw curves freehand on the corners. Drop the plywood into the truck bed.
Mark the corners of the plywood where it doesn't sit flat on the truck bed. Use the pencil to draw out the curves where your plywood doesn't fit. Take the plywood back out, make the cuts and fit again and cut again until it fits snug around your wheel-wells.
It doesn't have to be perfect. The plywood bed liner is there for abuse.
Always wear safety glasses when working with wood.