If you can find a 486 heatsink they are pretty close to an '040 heatsink. The clips may even grab the side of the '040 (Just make sure it doesn't short the pins).
I bought a used 3640 that came with a heatsink that didn't quite fit due to some bumps on the bottom. After grinding one of them off it fit perfectly.
I have no idea how hot it runs, but it doesn't crash in my A3000 desktop, even without a fan. The heatsink is even small enough to fit under the drive cage without a hole.
