About as cool as you can get...

All right folks, here you go! Now you can get that thick solid crosshair out of the way of your headshots.
Keep in mind that these changes will stick.

cl_crosshair_blue (0-255)
cl_crosshair_green (0-255)
cl_crosshair_red (0-255)
cl_crosshair_alpha (0-255)
cl_crosshair_dynamic (0,1)

Alpha of 128 will make a nice see through crosshair, and the dynamic setting of 1 will keep it adjusting to show you your accuracy. Enjoy!