Yeah, I asked about using it that way earlier, and was told it would not work.
Stuff that happens to smack into the bucket will do so harmlessly, but it's been game-mechanically established that we can't actively use it as a shield, even via using the bottom of the bucket as suggested (and yes, I did ask specifically about using it that way a while back).
I think we could likely get away with letting one or two bullets smack harmlessly into the bucket if that's all that's coming right at us, but with the density at our current position, there's no way we would get away with it. Most of the oni's stuff wasn't nearly this dense and we weren't able to actively use it as a shield then.