BEEF,
Blocked the lower grill about three weeks ago and haven't had any issues yet. According to my Scangauge, my internal water temps have not exceeded 200 degrees F, and that was on the warmest day yet this year (86 degrees F).
On most days, water temps run in the 180s and 190s.
I used galvanized sheet metal cut to match the grill opening, and toggle bolted the sheet to the honeycomb grill. (toggles on the interior). Hard to say how much FE improvement I have had, as I am just getting used to my Yaris as well. Figured it could only help as long as my water temps remain below boil point.
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