Best approach for a "removable" winter grill block on a mid-sized wagon?

[rojop64664]

Hey everyone,
I’ve been lurking around the forums for a few months now, soaking up as much info as I can. I finally decided it was time to stop just reading and actually start doing some basic mods to my 2014 Subaru Outback. It’s not the most aerodynamic beast on the road, but I’ve been trying to see how far I can push the MPGs on my daily commute, which is about 25 miles of mixed highway and backroads.
Lately, I’ve been really focused on my warm-up times. I’ve noticed that on these cooler mornings, it takes nearly seven or eight miles before the blue light on the dash finally goes off and the car feels like it’s actually running efficiently. By the time I’m at operating temp, I’m already a third of the way to work. I’m looking at building a DIY grill block to help trap some of that heat and get the engine up to temp faster, but I’m running into a bit of a design crossroads.
I’m planning on using some black Coroplast I scavenged from some old yard signs, but I’m torn on the execution. Most of the builds I see here involve zip-tying the panels directly to the exterior of the grill. While I know that’s the most effective for aero, I’m a little worried about aesthetics—my wife also drives this car occasionally, and she’s already skeptical of my "science projects."
Specifically, I’m trying to figure out the best way to handle the lower grill opening versus the upper one. In my car, the lower opening is much larger and seems to be where most of the airflow hits the radiator. If I block the lower section entirely but leave the upper open, am I risking a hotspot, or is the airflow behind the grill usually turbulent enough that it won't matter? I’m planning on monitoring everything with an OBDII scanner to make sure I don't overdo it, but I’d love to hear from anyone who has experimented with partial blocks on similar setups.
I’ve also been thinking about making the panels "quick-release" using some heavy-duty velcro or maybe some hidden clips, just in case we get one of those weird 60-degree days in the middle of February and I need to pull them off in a gas station parking lot.
Has anyone found a specific material or mounting method that looks relatively clean but can be popped off in under a minute without needing to carry a pair of snips for zip ties? I’m curious if anyone has noticed a significant difference in warm-up speed by only blocking the bottom half, or if it really takes a full-face block to see those gains during the winter months.
Looking forward to hearing your thoughts!

[freebeard]

I haven't had a lot of water-cooled vehicles, but I did have a VW Dasher diesel for a while. It was missing the stock flapper valve which was made of fiber board or something equally perishable. I've seen a line drawing, but can't find it at the moment.

The Dasher had a radiator offset to the side and a side-hinged flap in a duct that had a choke cable control. The only downside I see is needing a hole in the dashboard.