Okay! So, I grabbed a couple springs from the local hardware store, and found one that fits:
It fits nicely over the barrel. It's too long, but, I can cut it down. My only concern is that the coils are pretty thick, making it very stiff. Is that going to be a problem, if there's too much resistance? I can't imagine an o-ring has much resistance, and apparently people use those, so... Will it be okay?