Plan for this effective size and fit it horizontally (larger may be OK as long as its COLD outside). Mine stays in 'til daytime temps reach the low 80's (26C)
1-6x17" cardboard piece & +10 MPG! Yup no lie
http://www.insightcentral.net/forum/vie ... php?t=1970
Add just enough length to be as wide as the metal flanges of the radiator. Then use metal spring clip type paper clips to pinch it in place along the edges of the radiator. If you tuck the cardboard down tight at the bottom it should be held well enough (I hope

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HTH!