There are several methods for how to tighten the skin on the frame. Something close to this one was used by many seal hunters back in the day. You can also stretch the cloth and secure it with tacs at the gunwhales if you like and then shrink the top deck with an iron on wet cloth. It helps if you use some kind of treated twine, preferrably 1mm tarred, which doesn’t slip as much while you’re zig-zagging it. In Swedish skipper terms, it’s called “nätgarn”. The PU will glue it secure anyway.