The Baryon acoustic oscillations from Integrated Neutral Gas Observations (BINGO) telescope is a 40-m~class radio telescope under construction that has been designed to measure the large-angular-scale intensity of HI emission at 980--1260 MHz and hence to constrain dark energy parameters. A large focal plane array comprising of 1.7-metre diameter, 4.3-metre length corrugated feed horns is required in order to optimally illuminate the telescope. Additionally, very clean beams with low sidelobes across a broad frequency range are required, in order to facilitate the separation of the faint HI emission from bright Galactic foreground emission. Using novel construction methods, a full-sized prototype horn has been assembled. It has an average insertion loss of around 0.15 dB across the band, with a return loss around -25 dB. The main beam is Gaussian with the first sidelobe at around -25 dB. A septum polariser to separate the signal into the two hands of circular polarization has also been designed, built and tested.