I claim the ornamental design for a spin bag bracket, as shown and described.
FIG. 1 is a perspective view of a spin bag bracket, showing my new design;
FIG. 2 is a front elevational view of the invention;
FIG. 3 is a rear elevational view of the invention;
FIG. 4 is a left-side elevational view of the invention, the right-side elevational view being a mirror image thereof;
FIG. 5 is a bottom plan view of the invention; and,
FIG. 6 is a top plan view of the invention.
The broken line showing a suspended punching bag in the Figures is included for the purpose of illustrating the invention in use, and forms no part of the claimed design.