i put a used cfm airbox on, which is prolly the same, PB just puts they're name on sh*t, but i was concerned at first too, take a sharpie and mark the plastics along the frame where the mounting bolts hold it down, i made straight marks along there and cut it along that frame line, in the back i just cut olong the back line of the toolbox, get your new box mounted, then drop your plastics in place, i had to trim a little here and there for final , clean fitment.
you could prolly hold the airbox on top of the plastics and trace around it too, that'll get ya started, but cutting it all out along the frame looked better to me