Just for reference, tried to reserve Feb 1 earlier this month for a trip on Feb 17, nobinobi was full and had to shill 11,000 yen for a sleeper berth. But at least it was comfortable and not concrete I guess.
I've snagged one the day of myself too once upon a time in 2016, but honestly seems like a coin flip most times I ask.
Was easier with the Hamanasu and other blue trains with actual seats, since there were so many seats on board (though they also had a nobinobi carriage at times, but those were even harder to snag than the Sunrise I felt). But RIP in peace blue trains, and death to the Hokkaido Shinkansen for killing the last few sleepers besides the Sunrise.