Drop-Dreads. They require no space as they can just hang on to the ship and ride it like Slim Pickens rides a bomb.

Now that would be a good cartoon.

You know, that might be a good story arc.

Somewhere in the warp, is a Imperator-class Titan. Long ago, a daring RT decided to tempt fate by towing it with his Cruiser, through the warp. Either fate or ill misfortune befall the RT, as the chain broke during voyage. Lost to the currents, it drifts ever on. Some say they can hears it's voice in the warp when traveling close to the Heathen Stars. Some say its a tone full of sorrow and solitude hoping someone will find it, others a blare of wraith and fury at the fools who left. However, there is one thing they all agree on. It's voice yearns for what it was created for. War.......

An interesting potential Endeavour to say the least.

Given by an Ad Mech High Magos to the PCs in exchange for...( oh the possibilities ) They are sent to retrieve a titan lost to the warp due to ( insert sufficiently disastrous or otherwise maniacal incident ) Id say it was a Greater Endeavour ( given what the Ad Mech would pay/trade to have one of their God-machines back )