What
@DXX said definitely.
But you need to get the canvas rolled up nice and tight so it allows the clamp buckle to be in a vertical position which gives greater grip on the strap.
When you lower the roof pull all the canvas inwards and ensure none is trapped. Then try pulling the front piece of canvas back over the headrests and get a nice flat, crease free piece of material full width left to right then just roll it up nice and tight all the way to the front edge. Then thread the straps through the clamp and pull tight.