A tamper-proof enclosure for an electrical card, such as a high speed communications card, includes an enclosure in which the card is mounted. The enclosure has a wall with an opening, and a cup member is attached to the wall at the opening. A bus that is connected to the card extends through a passage in the cup member and through the opening in the wall. A security mesh is wrapped around the enclosure. The cup member is filled with liquid resin, which is also coated onto the security mesh. After the resin is cured, the resin in the cup member forms a plug that seals the security mesh from inner pressure when the enclosure is heated to an elevated temperature. The resin is preferably polyamide.