Trapping neutrons in a bottle is a real experimental technique. The speeds you propose happen to be too low, corresponds to a temperate of 0.0000000001 K. A few millikelvin (0.003 K or so) is achievable, which corresponds to speeds around 10 m/s.
Ultracold neutrons (that’s the technical term) trapped in a bottle behave like an ideal gas if their density isn’t too high. This gas is completely invisible.
You propose having around a kilogram of this stuff. Each neutron that decays releases a teeny bit of energy (“pinprick” was used upthread, but that’s too macroscopic an analogy. The average energy of the electron emanating from a neutron decay is in the ballpark of 10[sup]-13[/sup] joules.
But, a kilogram of them all together? The power output would be over a gigawatt, and the entire energy release (mostly done after an hour or so) would be equivalent to a 6 kton-TNT bomb, about a third the strength of the Hiroshima bomb. In just the first second of the reaction an energy equivalent of 7,000 kg of TNT would be released into the vicinity.
(For experts: this is neglecting the energy of the neutrinos, as one should.)
So your kilogram of neutrons would look… warm.