They make some large side boxes that sit above the bed rail that should hold a chainsaw easily enough. Another option would be to just mount a chainsaw mount on the top of a standard bed side box, if you mainly just take the saw with you when wheeling.
The one downside to shelves in the x-cab, is that nothing is secure, and anything with mass in an accident or rollover/flop, can become projectiles.
If you only haul lumber occasionally, another option would be to run a large storage box, and run one of the temporary/quick set-up lumber/ladder racks.
