Sure, here is an example of a bass guitar part that would work well with the chord progression I suggested:

| Cm | G/B | Cm/Bb | Am7(b5) |
| G  | G   | G     | G       |
| F  | F   | F     | F       |
| Eb | Eb  | Eb    | Eb      |

This bassline is fairly simple and doesn't have a lot of movement, which can help create a sombre feel. The notes are all from the C natural minor scale, which is the same scale as the chord progression. The bassline emphasizes the root notes of each chord, with the occasional descending chromatic line to add interest.

As for time signature, you might consider using a slow 6/8 or 3/4 time signature to enhance the sombre mood. These time signatures have a more relaxed, flowing feel, which can be a good match for a sad or melancholic chord progression.
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