This work presents the mechanical engineering design of the Diamond-II injector, which includes a new low-emittance booster with combined function magnets, modifications to the existing transfer lines and a novel storage ring injection scheme. The latter features two alternative schemes: the first scheme is based on a traditional four-kicker closed bump paired with a novel in-vacuum thin septum, combined with an in-air permanent magnet thick septum. This arrangement will be used for commissioning and initial fill of the ring; the second scheme, used for top-up during user operation, aims to deliver transparent injection and it is based on innovative strip line kickers.