Roll of hessian that builders use to cover newly laid concrete with, cheap as chips.
Then paint stripes,blobs,zig zag s etc with paint they found in the garage/shed on them. I cut them some hazel rods with forked tops for hide poles.
Once sited on rides or overlooking rabbit warrens, they were amazingly effective. Plus making them gave my kids sense of achievement, making something that actually works. Any slits required can be made with a knife,the frayed edges helping with the effect.