Well, you heard it from Igiss himself, 5000 maximum cargo hold. So, make it 5000 and only slightly less maneuverable and able to protect itself than the L-Train. Give it the same shields as the L-Train, slightly less armour, less maneuverabilty, and maybe only 4 turrets. And no CM dropper.
<span style="color:#FF0000"><strike>RETIRED</strike></span>
ON MY WAY OUT OF RETIREMENT