The double stroke roll. RRLLRRLL etc. Again, try to lock in with the metronome.
You can extend this by subdividing the metronome pulse differently. Quavers, triplets, semiquavers, one bar of each up and down the subdivisions and so on.
Remember to stay relaxed and try to make all strokes as even as you can. Use a mirror if possible, so you can watch your hands and sticks more easily. Repeat, using a metronome, for at least one minute, at a little under your maximum tempo.