The FREEDOM+ Surf decal antenna is designed to be installed on a flat surface, channels on the underside a surfboard would not provide a flat surface resulting in the electrodes not being applied correctly.
The electrodes are made from a 0.5 mm marine grade stainless steel called 316, and whilst extremely thin, they are still very ridged with little flexibility to bend around the channels. The flex cable is made from a thinner nickel which is more flexible and enables the antenna cable to bend around the rail.
Of course it depends on the channel design and where the channels are located, for example the attached image design would not work unless the bottom electrode fitted flat between the channel, the antenna would not bend to form around these channels.
We'd recommend chatting to your local surf shop before proceeding to purchase.