Feedthrough device (50; 150), for forming a hermetic seal around signal conductors in a signal conductor group (60; 160) with a group width. The device comprises a slotted member (52; 152) and a base (62; 162). The base defines a through hole (65) that extends entirely through the base along a feedthrough direction (X), and is adapted to accommodate the slotted member. The slotted member defines first and second surfaces (53, 54; 153, 154) on opposite sides associated with the feedthrough direction, and a side surface (55, 56; 155, 156) facing transverse to the feedthrough direction. The slotted member comprises a slot (58; 158), which extends along the feedthrough direction through the slotted member, and opens into the first and second surfaces and into a longitudinal opening (59; 159) along the side surface. The slot extends transversely into the slotted member up to a slot depth at least equal to the signal conductor group width.