An implantable casing (200, 300, 400, 500, 600, 700, 800, 900) for securing an implantable adapter connector module and an implantable medical device together within a subcutaneous pocket of a patient, the casing comprising: opposing first and second plates (201, 202, 301, 302, 401, 402, 521, 522, 521D, 522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) spaced apart from one another by a gap (250, 350, 350D, 530, 530D, 630, 730, 830) configured to receive the adapter connector module and the medical device; and a sidewall (203, 303R, 303F, 303-DR, 303-DB, 403, 703, 703D, 803) joining the first plate (201, 301, 401, 521, 521D,621, 721, 721D, 821, 921) to the second plate (202, 302, 402, 522, 522D, 622, 722, 722D, 822, 922) to define the gap (250, 350, 350D, 530, 530D, 630, 730, 830), wherein the gap (250, 350, 350D, 530, 530D, 630, 730, 830) between the first and second plates (201, 202, 301, 302, 401, 402, 521, 522, 521D,522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) has one or more entry openings defined by portions of the first and second plate (201, 202, 301, 302, 401, 402, 521, 522, 521D,522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) that are not joined together by the sidewall (203, 303R, 303F, 303-DR, 303-DB, 403, 703, 703D, 803), and wherein the first plate and the second plate (201, 202, 301, 302, 401, 402, 521, 522, 521D, 522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) are spring biased toward one another such that, when the device and the adapter connector module are received in the gap, sides of the device and adapter connector module are in confronting engagement with, and secured by, the first and second plates (201, 202, 301, 302, 401, 402, 521, 522, 521D, 522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) and a connector port opening of the adapter connector module faces generally toward one of the one or more entry openings. opposing first and second plates (201, 202, 301, 302, 401, 402, 521, 522, 521D, 522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) spaced apart from one another by a gap (250, 350, 350D, 530, 530D, 630, 730, 830) configured to receive the adapter connector module and the medical device; and a sidewall (203, 303R, 303F, 303-DR, 303-DB, 403, 703, 703D, 803) joining the first plate (201, 301, 401, 521, 521D,621, 721, 721D, 821, 921) to the second plate (202, 302, 402, 522, 522D, 622, 722, 722D, 822, 922) to define the gap (250, 350, 350D, 530, 530D, 630, 730, 830), wherein the gap (250, 350, 350D, 530, 530D, 630, 730, 830) between the first and second plates (201, 202, 301, 302, 401, 402, 521, 522, 521D,522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) has one or more entry openings defined by portions of the first and second plate (201, 202, 301, 302, 401, 402, 521, 522, 521D,522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) that are not joined together by the sidewall (203, 303R, 303F, 303-DR, 303-DB, 403, 703, 703D, 803), and wherein the first plate and the second plate (201, 202, 301, 302, 401, 402, 521, 522, 521D, 522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) are spring biased toward one another such that, when the device and the adapter connector module are received in the gap, sides of the device and adapter connector module are in confronting engagement with, and secured by, the first and second plates (201, 202, 301, 302, 401, 402, 521, 522, 521D, 522D, 621, 622, 721, 722, 721D, 722D, 821, 822, 921, 922) and a connector port opening of the adapter connector module faces generally toward one of the one or more entry openings.