Here's some photos of my Gipsy's canvas top 

You'll notice the aluminum sheet over the back part of the roof, I'm not sure if this is just for the antenna plane or if it supports the canvass as well

 

 Here's another Gipsy showing details of the frame and some measurments

 

  

Here is a Gipsy with the half cab and a canvas rear setup. I'm not sure how the canvass would attach so that water won't leak at the front of the box