How about using a forstner bit on a scrap piece of wood, say a 1x6, center the hole end to end and leave the board long, about 24 inches. Position the hole you just drilled on your truck floor where you want it. With both knees on the scrap board drill between your legs. Use the hole you drilled to guide your bit, work slow. The pressure from the scrap should keep the wood from underneath from splintering. Gauge your depth by eye. I used this when making a tool rack. I drilled the holes for screwdrivers, when I wanted it for handles to sit in a recess.
