Getting to the smaller end of tentacles, these were done with printed mould negatives, allowing me to print them out of a formlabs machine (shown below) then simply fill with silicone to create both halves of the mould.

the two silicone halves could then be pressed together with fast cast resin inbetween, venting channels allow excess resin to escape easily. simply pulling out the piece as a sheet and snipping the flashing off.