Here we go, a summary of all the somewhat presentable stuff I found regarding cargo containers.
http://fleetyard.net/temp/trek/containers-summary.png
First of, the standard container, a rather simple 200 meter long cylinder with a 20 meter radius. They are really quite remarkably huge, but I suppose you need that for interstellar transport to be meaningful. I suppose there could be shorter variants that only have the front and aft connectors, and not the full topside connector assembly.
Next, my interpretation of the
Vanguard class from SFC (I can't find any pictures of the SFC drawing online, but I'm sure you're familiar with it). It's basic premise is of a War-era (fuel?) transport continuing life as a 23rd century robotug.
Below that is my Class T 'tugboat', mostly an inner system craft that can tow (unpowered) containers around. There are containers that can fly themselves around (think of FJ's starliner version), but most are 'dumb' containers.
And there we have the
Imhotep as well and some variants: top to bottom, there's an dangerous goods transport, a 'tandem carrier' and a tractor beam based general tug. As I said, I'm thinking of slimming it down, and old concept of that idea is presented as well.
Another concept I had, but never worked out, is taking SFC's
Watt class and turning it into a tandem container carrier. Possibly even carrying 4 containers as in
Reverend's configuration.