An occlusion device (160, 260) for transcatheter closure of an arteriovenous fistula or tract connection, comprising: an implant comprising a shape-memory metal frame that is resiliently deformable into a radially compressed configuration for transcatheter delivery and radially self-expandable to an implanted configuration, the implant comprising a venous end portion, an arterial end portion (164, 264), and an intermediate neck portion (166, 266) between the venous end portion (168, 268) and the arterial end portion; the implant configured to be implanted across an arteriovenous fistula or tract connection between an artery and a vein with the arterial end portion positioned in the artery, the venous end portion positioned in the vein, and the neck portion positioned in the fistula or tract connection; wherein when implanted, the arterial end portion of the implant is radially enlarged relative to the neck portion such that the arterial end portion forms an arterial skirt that contacts an endoluminal wall of the artery and blocks blood flow from the artery into the fistula or tract connection; the implant further comprising a longitudinal guidewire channel (162, 262) extending through the venous end portion, the neck portion, and the arterial end portion of the implant, wherein the guidewire channel allows the implant to be delivered over a guidewire and the guidewire channel comprises an elastomeric cylinder placed within the guidewire channel that forms the inner face or core of the guidewire channel and can radially expand toward the longitudinal axis of the guidewire channel to occlude the channel when a guidewire is removed in use; characterized in that the neck portion comprises a plurality of circumferential corrugations that are elastically deformable by tensile force applied on the implant to cause the corrugations to pull out and lengthen the neck portion. an implant comprising a shape-memory metal frame that is resiliently deformable into a radially compressed configuration for transcatheter delivery and radially self-expandable to an implanted configuration, the implant comprising a venous end portion, an arterial end portion (164, 264), and an intermediate neck portion (166, 266) between the venous end portion (168, 268) and the arterial end portion; the implant configured to be implanted across an arteriovenous fistula or tract connection between an artery and a vein with the arterial end portion positioned in the artery, the venous end portion positioned in the vein, and the neck portion positioned in the fistula or tract connection; wherein when implanted, the arterial end portion of the implant is radially enlarged relative to the neck portion such that the arterial end portion forms an arterial skirt that contacts an endoluminal wall of the artery and blocks blood flow from the artery into the fistula or tract connection; the implant further comprising a longitudinal guidewire channel (162, 262) extending through the venous end portion, the neck portion, and the arterial end portion of the implant, wherein the guidewire channel allows the implant to be delivered over a guidewire and the guidewire channel comprises an elastomeric cylinder placed within the guidewire channel that forms the inner face or core of the guidewire channel and can radially expand toward the longitudinal axis of the guidewire channel to occlude the channel when a guidewire is removed in use; characterized in that the neck portion comprises a plurality of circumferential corrugations that are elastically deformable by tensile force applied on the implant to cause the corrugations to pull out and lengthen the neck portion.