List of Good Aftermarket Nav unit/Alternative HU inside:

SoCal 370Z

Here are the dimensions, but please note that this is not standard rectangular box, but an opening that is wider and higher at the front and then tapers down on all four sides to narrower dimensions at the back:

Front Width: 6 3/8'
Front Height: 3 3/4"

Rear Width: 5 13/16"
Rear Height: 3"

There is a 45 degree chamfer at the back of the box at its bottom the cut into the bottom dimension approx. 3/4". I hope this helps.

AVIC-F500BT Dimensions (W × H × D):
184.8mm × 98.1mm × 29.7mm
(7-1/4 in. × 3-7/8 in. × 1-1/8 in.)

Depending on how you want to mount it you might choose to go this route when they produce their mounts for the 370Z: ProClip

Take a look at the pictures at the forum's album, by zed-er, named: 370 Launch
It should not be long before an aftermarket replacement inset is released.