Keep in a cooler put down a layer of crushed ice, tons of kosher salt, then bunker, then salt, then ice, then salt, then bunker, etc. Make sure you use tons of salt and crushed ice and I've had them good to go for 4 or 5 days.
Catch fish until you don't have any bunker left to keep fresh!
Serious now, though, I agree with the others that avoid freezing and keep on ice is the way to go. It is also worth it to get a high quality cooler that way you're not buying many bags a day just to keep fish fresh. The investment in a good cooler should pay off for the ice expenses. Or you can do what i do: snag a bunker, and throw him back with a hook in him attached to a line on a pole.