My Spin 99 is mounted underneath the fuse floor just forward of the landing gear this provides a nice short run between the motor and batteries. Cooling is not an issue as long as you cut some slots to the rear of the chin cowl to draw some of the air through. Remember that you want to direct as much of the air coming through the front of the plane to your key components (ESC, motor & batteries). I made up a few depron structures to direct air to each of these components and have found that everything stays quite cool.

Air outlets are incredibly important. I've molded two 50x100mm scoops in the fuze out of CF in the belly of the aircraft just behind the wing tube. These vents are not just a hole in the belly but scoops that create negative pressure in the fuse to assist with cooling (post 629 from this thread has pictures of these). Work great and look cool!

Hope this helps, Simon