Since cutter modules can be used as fuel tanks to directly feed the J-Drive (The Modular J-Ships for Classic and GT do it) one can safely assume that any rugged enough container can. Normally one would want to build the containers rather large so that 1 Container/Jump is the norm for ease of mounting and efficient operations
But the Typ TJ does not have to be efficient, it must be abel to pass a casual inspection. And since it is an Imperial Spyship one can also assume it has the necessary gear to change manifests, fake transponders etc. as well as a crack crew(1). And they are described as "working the backwaters, posing as slow tramps"
So the cargo in the "fuel holds" is always labeled "shipped to next planet after this one" and never needs to be unloaded, allowing the fake containers to be rather sturdy. Add in multiple but equal-sized holds, each build to fuel-tank standards (done on the TI just as well, claim "ease of building was more important than price") and you have your fire protection.
The fake cargo will be made to look like 40ft/8dton standard containers stacked neatly (Imperialines is a small line so "We keep ourselfs to better standards than the average tramp") and you might even get away with making most of the container block "one large piece"
The idea of different TL's also has merrit, even more if used with the article on "Additional parts for spaceships" in MTJournal 3 that includes such nice stuff as a pop-up turret. Using a more compact drive and/or reactor allows you to hide stuff within the reactor housing.
And since this is a "build from the keel-up" spyship rather than a "I made some modifications myself" Han Solo field-upgrade, the camouflage should be high-quality.
(1) These are the Emperors own after all. Most likely the first four ships where named Artos, Aramis, Portos and d'Artagnan