That's a nice temp bump up, though again not enough to prevent ICE startup in cold weather - yes?
I did block the front vents this year (with black foam pipe insulation, plus a few black twist ties). I also used some self-adhesive foam insulation (about 1/2 x 3/4" profile, as I remember) on the bottom edge of the hood to better seal over the headlights and across the front. I read that Ford made a change for MY 2014 to that hood seal, but have not seen what they did.
I tried a little 200 watt heater in the passenger footwell today (ambient about 20 degrees). It could only bump the cabin temp by 12 degrees after 2 hours. (Heater is controlled by a z-wave switch which in turn is controlled by Amazon Alexa; temp is reported by an Acurite sensor (on the drivers seat) via their internet bridge. This way I can control the heater and monitor the results from my study.)
I'll try a 500 watt heater tomorrow. It would be nice to run a single z-wave switch powering both a low watt heater along with the block heater But that wastes a bunch of heat/energy dissipated out of the slowly warming cabin. So maybe the way to go is separate switches and a high watt quick heat of the cabin. That may require different heater placement.