I just added a roof rack to my 2010 prius
as well as a Thule Weekender
I figure the roof rack is worth about -1 or 2 mpg on open highway
an the roof box is worth about -4 to 5 mpg
(but it wont be on my car all the time)
mine is similar to this
*** Thule cargo box ***
what is your local craigslist?
I'd suggest a full roofrack kit (about $500 new)
Thule or Yakima are both great products
I'd suggest spending no more than necessary for a super smooth looking one since it's just gonna be for occasional use
good luck
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2005 GTO 6.0 V8 (600hp)
1991 Geo Metro 1.0 3/5 base
2010 Prius II (replaced 2001 Tahoe 5.3 V8 April 2014)
1994 F-350 7.3 TurboDiesel (Rollin' Coal)