The LSI SpaceJunk Cargo Bays are designed to carry payload items internally. Models are available for 1.25m, 2.5m and 3.75m cargo. Attachment nodes are located inside each end.
The part was created by Legal2k. He has granted me permission to update it for compatibility with KSP version 0.18+ and make it available for download.
1.01 – Initial re-release by RoboRay. Direct adaption of Legal2K’s 2.5m cargo bay, compatible with KSP 0.18+.
1.02 – Repositioned attachment nodes for flush-mounting of interior and exterior parts. Slightly enlarged 2.5m cargo bay to allow placement of RCS quads on 2.5m payloads. Added 1.25m and 3.75m versions of the cargo bay. Renamed folder for 2.5m cargo bay.
Extract the Parts folder into your KSP folder.
NOTE: v1.02 required renaming of the original part folder for consistency with the newly added parts. If you have saved .craft files using the old v1.01 part, you should install this update, edit your saved craft to substitute the new 2.5m part for the old one, then delete the KSP/Parts/[LSI]CargoBAY folder. If you have craft in flight using the v1.01 cargo bay, do not delete this folder until those missions are completed.
How to use
The bay doors can be operated by right-click menu or by assignment to Action Groups in the VAB or SPH.
If you are having trouble attaching payloads inside, try shifting the camera position so that you are inside the bay looking out the open doors. This allows easier access to the attachment nodes, especially if the part you are trying to add allows surface attachment (like unshielded docking ports).