The easiest way probably would be to use standard material.
Usual side lengths when it comes to isolation material such as
wall panels and others are 300, 600, 1200 and 2400 mm.
Two kinds of material are common; glass wool and rock wool.
Rock wool drissles off more, but glass wool makes your skin
itch if you get it on your body. From an acoustic point of view,
they´re both good. An american company called Armstrong
makes panels like these.
A nice box could be built with six panels that are 600x600 mm.
In the front panel a hole maybe 200-300 mm should be made
(or any size that you find convenient. Try out the smaller sizes
first). When you make the hole, dusty stuff from the panel may
occur. Make hole outdoor!
The panels have to be assembled somehow,
maybe like one of these options:
* You could build a wooden box and glue the
panels on the inside.
* From the place where you get the panels,
you most likely can get some metal
profiles that could hold them together.
* Just gluing them together isn´t a good idea.
They´re not strong enough for that, I think . . .
* If you use thick material like 100 mm, they
just might be able to stand by themselves.
Place the box in a book shelf at a suitable level.
Throw out books that only your parents read.
Don´t tell them that I said that!