Notice how modal this exercise sounds. There will be plenty of parallel 5ths in the left hand but it's OK because this style permits these intervals which otherwise would be forbidden in tonal writing.
I have completed the first measure for you. If you want, you can change the mode and experiment with adding Bb or F# everywhere you find these notes. In fact, you can add strange combination of accidentals, such as F# and G#. This will create Lydian-Mixolydian mode from D (raised 4th and lowered 7th scale degrees).
Feel free to add some pedal points, too. Here is the PDF file for printing.