5 days Sprint by Jake Knapp? Have you tried to apply this to your product prototyping?

Yes. It's not as easy or time-boxed as it sounds. It assumes you've got the resources of Google to find a bunch of people to talk to within 1 day. That work takes a ton of time. If you can do that, then the 5-days is possible and awesome to do. (I worked within Google and participated in one led by someone junior - let's just say the parts where you talked to customers/users was... skipped.)