Neither Garmin not BRE but, looking for a temporary place for the transducer for my Clipper depth sounder, I used a lump of blutack and placed it in one of the lockers under a seat, about 40cm from the keel plank. As it enters its third season, it's still working well, still happily bonding with the original piece of blutack.
The BC hull has quite a flat profile so not so important to place centrally.