Fixing canvas to a UPVC frame?

Rather than pay out a fortune for roof blinds, we have cut and sewn pieces of canvas fabric to fit each section of roof panel on our conservatory. We have used self-adhesive velcro to stick each panel in place. It looks great until the sun heats up the conservatory and the velcro starts coming away from the UPVC causing the panels to hang down. Is there anything we could use to hold the panels in place, we have thought of upholstery gimp pins (small tacks) but not sure whether these can be hammered into the plastic and whether they will hold.
Any ideas?