Cargo will be categorized into three types: pressurized, unpressurized and 'chemical'. Pressurized cargo would involve elements that require some sort of provision for atmosphere and so must have a pressure-tight container. Unpresurized cargo is typically machinery, construction materials or tools that do not require pressure or gas for protection.
Chemical transport is a third category because it can be accomodated by enlarging already existing tankage on the lander. In some cases, with dissimilar chemicals, it may prove feasible to partition tanks with a bulkheads to transport chemicals without the need for a sepaqrate tank and support structure.
Pressurized cargo modules naturally integrate into a Base's overall structure as airlocks, storage modules or other habitable volume. Conceptually, if they are designed for this second-order use, it would be a requirement to outfit them with power and ventilation access that plugs into the rest of the Base. A simpler approach of outfitting an inert 'tin can' for use as habitat post-landing saves no cargo mass as it requires transporting the power and ventilation equipment nmentioned as a separate item in the cargo manifest. Worse, valuable time is consumed in having to assemble the equipment - and that not until after the module has been cleared of its contents and is accessable to the crew.
Off-loading cargo is facilitated if it is not very high off the ground. Generally, this points toward arraying the engines around the payload module/pallet and mounting the propellant tanks above the payload. this also has the benefit of lowering the module's center-of-gravity to a point at or near the thrust plane at the moment of touchdown. Once landed, the cargo can be off-loaded by backing up a trailer under the cargo, lowering the module/pallet, then driving off. Ideally, the tankage and other elements of the cargo lander are utilized in the base in some manner. Of course, any residual propellant is scavenged.
There is good reason to deliberately plan for SLIGHTLY under-sized cargo. Under-size is defined here as less than the estimated delivery mass capability built into the launch vehicle/lander system. The idea is to plan to include a standardized mass of either hydrogen or methane as 'ballast' which is subsequently used in processing ilmenite for the oxygen content. This allows growth in the Base's water supply without the inefficiency of actually allocating payload mass and volume to water.
All of this evolves to a cargo system with the following characteristics:
PRESSURIZED MODULES designed for second-order integration into the habitat are no less than 2.5 meters in one axis and no less than 1.5 meters in at least one other axis. Variations can include between one and six pressure hatches, the latter being a universal hub connector. Module structuring must accomodate not only the minimal pressure detailed for the cargo but also the pressure of the Base's atmosphere. Very likely this will be a rib/stringer and panel assembly. Power and ventilation elements are installed pre-launch. Pass-through ports for gas fluid, power, communications and other conduits are incorporated into the hatch aperature, which is also the seal ring for the connection to the rest of the Base. The hatch aperature is 2.0 meters long (vertical axis) and 1.5 meters high (horizontal axis) The actual hatch is 1.75 meters long.
Mass for the empty module alone, assuming lightweight composite material (@1.44 g/cm2)is estimated here at 220 kilograms.
Importantly, ALL cargo missions, regardless of cargo type, should integrate 5 m2 of photovoltaic arrays into the spacecraft structure. These would be integrated into the Base's power system. This allows growth in power capability of about 1 kw per cargo mission.