I have a home-made canvas for my van with a great big oversize front screen.  However, my friend's wife, who made it when it was their van,  was limited for time, so there is no zipper to unzip the screen for access to the luggage rack. 

How possible do you think it would be for an owner, or a shop with expereince, to add this zipper, (and possibly side window screens with flaps)  with the canvas still on the van? If I was to pull the entire top, I figure I might as well as add a new 3-window canvas and sell off this top. 

BTW - she did an awesome job on the canvas.  

John