Hi,
You can buy Coroplast at sign maker shops. You won't need very much -- I would even just try it out using duct tape. Yes, start keeping track of the coolant/water temperature on the SGII. It should be pretty constant near the temp for the thermostat -- 180F or 190F are common, I think?
Check with others with pickup trucks -- I think you'll be able to block all three of the upper openings.
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