Should have said densest substance on Earth. Fill it with neutron star and you’ll be approx. 30 trillion kg overweight.
Had to post this correction before @neiltyson popped in with an "Actually..."
I don't have anything to plug, so just try to be a little bit better than normal if you happen to think of it, but don't make a big deal out of it--no one's asking you to be a saint.
The interior dimensions (8 5/8" x 5 3/8" x 1 5/8") are ~75.333 in^3.
If you filled the box with pure osmium, the densest substance known to man, it would weigh ~61.48 lbs.»