Raising the firewall to windshield height will also make a defroster duct.

This aluminum angle completes the top-of-firewall HVAC plenum; it's level with the windshield's horizontal flange. The front top panels will rest on the forward half of the horizontal flange with the rear top panel on the rear half. I'm yet sure how, but somehow the rear panel will be removable by releasing a couple of concealed catches or fasteners.