Fish funeral suggestion???

We have a beloved old betta, about 4 years old, that we "rescued" from
a coffee-cup-sized bowl in an office about 3 years ago. He's showing
dropsy-like symptoms: swollen, whitish belly, lethargy, barely eats,
hangs motionless in one spot all day. He's been this way about 3 weeks
now. Considering his age, I don't think he's going to make it, even
with antibiotics and water changes.

Burying a fish down in the dirt just doesn't feel right. Do you agree?
Of course, no way we're going to flush him down the toilet.  We do
live near a large lake, and we've though about putting him in a
"coffin" and putting him in the deepest part (about 180 feet). 
Anybody have any ideas or suggestions? It's amazing how attached you
can get to a living thing over time, even a fish.

RM