For something that small I would just use body filler or epoxy mixed with micro balloons. If you run a strip of tape 3/8" from the joint on both surfaces, press the filler in tight, and use the end of a plastic spoon to smooth it out you'll get a nice evening radius that requires little sanding. If the joint still needs to come apart, lay a sheet of wax paper on the surface you don't want the fairing sticking to. Follow the same procedure and you have a removable piece still.