You could just buy deep out of money SP500 puts expiring in 1+ year. That way you would be "insured" against the bubble popping.

The thing is, every dollar you spend on insurance is a dollar (and its interest) you lose. Furthermore, we don't know when it will pop. 1 year? 5 years?

The more reasonable solution is probably gradually reduce exposure to US markets by selling SP500 shares and turning to Europe and emerging markets ETFs. No need to cash out 401k.