Here are some important points to remember:
1. The texture can be similar to that of the Toccata.
2. You can use Fortissimo registration - with some of loudest stops in the manuals and in the pedals.
3. The right hand can play a fast figuration in sixteenth notes.
4. This figuration can consist of two juxtaposed intervals of perfect fifths, one upward and one downward (for example F-C, fingers 2 and 5 and G-C, fingers 3 and 1).
5. The left hand can play melodies in quarter notes which are build from open fifths in step-wise motion (D-A, E-B, F-C).
6. The pedals can double the lowest note of the left hand as the reinforcement.
7. Start by a short introduction of this figure a few times while holding an open fifth on the main note of the mode.
8. Don't worry about the names of the modes, simply say to yourself the number of accidentals.
9. Change the number of accidentals randomly.
10. Change the accidentals every 15 seconds or so to keep it interesting and colorful.
11. Aim for at least 3 correct repetitions in a row of this exercise.
Here's my example (with soft stops).
Feel free to post your example online and share the link in the comments here. Also post time and the number of repetitions to comments.