as experience with airplane models i can tell you that shrink plastic that its born for those its expensive compared to other solutions, basically any plastic sheet will shrink to a certain point, window covering shrinks a lot and could be fine but if you want something beefier and cheap just go to any fabric store, grab a sintetic fabric (and there are of all color and thickness) and use that, you can find lots on clearance or end rolls. Get one of those scissor that cut the fabric in a zig-zag pattern (inexpensive at dollar stores quite expensive if pro taylor stuff) it will help with the look (a must for airplanes models that want to reproduce the real fabric coverings but nice on a kayak too) and youll get less crazy glueing the edges with threads falling off the fabric. An iron on cotton setting, the highest, will shrink the fabric perfectly.
With thick fabric be careful to check for deformations of the frame, if look its important careful when you step with the iron onto the frame, there the fabric cant move away and it can scorch a bit, its just cosmetic, depending on the fabric you could melt through so be fast and dont press too much check on a scrap first. An hair drier works too but the air pressure doesnt help sometimes hence ironing.