Ok, I'm bored, so I'll keep going.

If you've got 28 inches of width, and three sheets if 3/4", your internal width is 25.75.
Height is 15 minus two sheets of 3/4, or 13.5.
Depth is 17 minus two sheets of 3/4, or 15.5.

25.75 x 13.5 x 15.5 = 5388.19
5388.19 / 1728 = 3.12 total
3.12 / 2 = 1.56 per chamber