I don't have a D600 but the D800's do that too, especially the D830, I lost a nVidia GPU I assume it came 'unfloated' due to heat in a D830 whilst 3D rendering years ago... anymore ANYTIME I use a D800 I place a 'fence' under the back feet to raise the laptop up 1/4-3/8 inch for better air flow underneath... I use anything I have some wooden trim then glue shelf liner on the bottom so it does not shift but I also use foam strips like what you get in packaged crap... If you have/get a Dell Dock that also does the job & nice to have all connections to the dock not the laptop...
Now when doing GPU heavy tasks I have a desk fan or other fan blowin' right on the machine as since it's a metal laptop it helps quite a bit of course I blow out the internal fan regular as it will build with dog dander & chemtrail dust...
In tighter spots I have a few micro fans I removed from a stereo that put out some velocity in 12v... I have these hooked to our 12v system that's run off solar cells outside (I wired 12v thru entire house years ago)...
Hopefully that helps...