Here's someone else doing the cheapo do-it-yourself tarp solution, a $30 "Ozark Trail 9? x 9? Dining Canopy". http://bodeswell.org/2010/04/09/el-cheapo-20-awning/. I still have got to rip my fancy awning off someday and do something like this.
BTW the shadyboy awning case is narrower than the PH top in the back. I have one I'm planning on mounting for a rear awning. Your solution looks great for a lot less $$$.
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Seth Hatfield
'05 EB350 6.0 4x4
Homebuilt Interior
Santa Barbara Ca