I highly recommend the UVS100 Heat Shield. It isn't the lightest, but is custom fit to the NSX windshield, a little thicker, and therefore highly effective, especially if you have tint on the sides and rear of the car.
Plus if you get a decent bag/case for it and you don't sit super far back, there's room to stash it behind the driver seat fairly easy (getting it in and out of there just requires sliding the passenger seat forward enough so you can tilt/rotate it in/out).