I have written a bass counterpoint part that goes with the last verse (joining in guiltily with the lead at key points), or can be sung separately. My intention is to record the song (soon!) in a bluegrass gospel style.
The rhythm is mostly defined by quarter notes.

If you want to sleep at night
You must give me tuna, tuna in large doses
Tuna, tuna, all the time
That's just what the world owes us.
And if you don't I'll wake you
Long before, before the rooster's crowing
I know you would not like that
How can I keep from purring?