The traditional flat track frame (and trust me: I've studied quite a few) uses a turned up metal loop as a connection point for the rear bodywork. Or in the case of a Woods frame, the rear frame loops around the bodywork. The loop makes the rear frame stiff and light.
I knew however that I needed an easy way to fix the rear light, reg plate and indicators on my street tracker, so I opted for a slightly different rear frame with a small "shelf" at the back. Not quite as light but it makes for a convenient place to put some of the electric gubbins that are not needed on a race tracker. None of these components are visible once the rear bodywork is fixed in place and the "shelf" protects them from rear wheel spray. As you can see from the image, the horn was relocated backwards as well.