I use a chain. around the seat rail to the seat stay. It is a lighter gauge than a bicycle chain but the same kind of links, only smaller. I think it was a small-motor chain will try to post a pic. Also a company used to make a seat post "anchor" I guess using a star nut, which had a cable connected to the water bottle cage screw inside the seat tube. Wild but effective. If u r handy u cld fab that up easily.