Simply a box with a hole in it, this shows that the "hole" is a perfectly absorbing blackbody (and so looks black in the visible, at room temperature), regardless of the contents or composition of the box.
As a demonstration, open up the box, and students
will see that the inside walls are actually white.
This demo can be found in cabinet D on shelf 1 of the UCSD PUL Demo Room
(WLH 2015).