Two guys in our club tow banners and both hook to the bottom of the plane near the CG. They're configured as Bob Moore recommends, made of mesh with fabric letters, a few dowel stiffeners and small weights crimped along the bottom edge.
They take off with the banner furled, unroll it in flight and cut it loose to land.
Occasionally the banner comes out but doesn't unroll, just trails along at the end of the leader all rolled up, but usually a low pass to bump it on the ground will knock it open, which makes a better show!