dont discount gravitational waves as a thing!
You may not need magic mass, if you can create a standing sinewave. Space clearly has some elastic properties that are predictable, otherwise gravity waves would not propogate in a comprehensible manner. By understanding those elastic properties, one can create spectacular effects with small inputs.
Take for instance, the FloWave experiment. It uses a great big swimming pool, essentially, and some reasonably small solonoid driven paddles all around the circumference. By carefully manipulating when and how much each paddle moves, very precise waveform reinforcement/dissipation can be accomplished. One of the more spectacular displays is the "concentric singularity" routine. It basically causes a collosall 8 foot spike of water to erupt from the center of the pool, straight up.
https://www.youtube.com/watch?v=RHTcSKkUU8UGravity waves ripple in all three spacial dimensions, so all of space is the "surface" of the pool. According to some sources I have read, creating very low energy gravity waves is very easy. You just need a rotating barbell shaped mass going end over end. Putting lots of those in either a spherical or hemispherical arrangement, with a computed point as the desired location for the reinforced wave, you should be able to create a momentary gravity well of very significant intensity, and do so over and over and over again.
The vessel will have mass, and the induced gravity well, and the ship will be attracted to each other. The well will dissipate as space returns to normal curvature, but the ship will continually fall forward toward each subsequent spike.
The ride would be a bit bumpy, but it would be reactionless drive.
Another interesting prospect is the creation of a galactic torus "racetrack" megaconstruction. Basically, small ringlike structures sit along the path of the resulting torus, and they impel standing gravitational waves down toward the next ring, which reinforces the incoming wave and slightly redirects it. This creates a torus of fast moving spacetime, through which an object can be propelled very quickly down the "track". (possibly even at ftl) By carefully plotting your intersection with the torus, you can predict where you will pass out of it later.
Gravity being waves is mega awesome.