[LS short box]

I've always bought the generic bed mat and cut it to fit. The wheel well contours are similar for most bed mats.
Couple of tips.
As they say measure once cut once. I start with the front and cut it to fit the wheel wheels. Then trim the back as needed.
I lay the bed mat out on a piece of plywood after I've made my cut lines. On the 8 foot length side. Then using a piece of 2x2 I screw it down to the plywood trapping the bed mat between so it can't move as I cut it. Using really (brand new) sharp razor knife blades I cut it as needed. Takes a few passes with the razor knife but the 2x2 makes for a nice clean cut line.