This is the direct descendant of the Arrow Class SSTO I used in my (Stalled) Affordable Space Program Challenge entry attempt, it's currently being used in my (also Stalled) SSTI Challenge entry attempt.
The Fulmar class SSTO can carry cargo in one of two locations. For compact (around the size of a Rockomax x200-16 fuel tank) cargo weighing less than 10t, there is a rear cargo bay, located between the engines. For payloads too large for the cargo bay, the Fulmar features a dorsal cargo mount. I have managed slightly heavier payloads on the dorsal mount, but the Fulmar caps out at just over 11t.
Given its awkward CoM/CoL offset, the Fulmar can be... Temperamental, in flight. To maintain stability, it is recommended that all Liquid fuel be moved as far forward as possible. I tend to lock all but the rearmost MK 3 fueslage and then unlock the others progressively as they drain.
update: As of 05/07/14 I've begun adding example payload sub-assemblies to the download, for those that want them. I intend to keep tacking more on until it has a decent variety of parts to build stations or interplanetary craft.
Current example payloads:
A basic station core, and a set of empty fuel tanks. (how you fill these on-orbit is left entirely up to you)