I know the YN-300III comes with a diffuser sheet but for me, the light was not soft enough on full power using the supplied plastic diffuser piece.
You will need 12 pcs 5x3 mm cylindrical magnets to assemble the frame:
- push 4 magnets to the holes at the corner of the frame
- cover the frame with the diffuser material
- put 4 magnets to the corners to fix the diffuser on the frame
- remove the plastic cover pieces from the corners of the YN-300III led light which hides the screws
- push in 4 magnets to the holes on the light's frame
I cut out a piece from a shower curtain as diffuser material.