The Tripod Mount Ring B would be too small for your EF 300mm F/2.8 L IS lens. It is made to fit the MP-E 65mm f/2.8 1-5x Macro, EF 100mm f/2.8 Macro, and EF 180mm f/3.5L Macro among other lenses.
My EF 70-200mm f/2.8 L IS II uses the white version of the Tripod Mount Ring B which has an inside diameter of 78mm.
Tripod Mount Ring A (EF 70-200mm f/4 L IS and EF 70-200mm f/4 L, EF 200mm f/2.8 L II) is even smaller than Tripod Mount Ring B.
Tripod Mount Ring C (EF 70-300mm f/4-5.6 L) has an inner diameter of 82mm.
Tripod Mount Ring D (EF 100mm f/2.8 L Macro) is even smaller than Tripod Ring A.
The EF 300mm f/2.8 L IS has been out of production since the EF 300mm f/2.8 L IS II was introduced in 2011. As early as 2012 the ring was no longer available as a replacement part. Even third party knock-offs of the tripod ring have dried up by now.
Canon does still list the EF 300mm f/2.8 L IS on the list of lenses currently repaired by Canon Professional Services, but they may no longer fully support servicing the older version due to lack of parts availability. If you're a member of CPS it couldn't hurt to give them a call.