Splitting the left pentagon into a trapezoid + rectangle (each with a height of 6.25" after wood thickness)... then multiplying it by the width (11.5").... I got 0.904 cf before displacement.
If it's a 10" sub, I'd guess 0.1-0.15 cf displacement. Meaning it's 0.8 cf sealed IV box after displacement.
Edit: If your brother is an engineer too, he probably suggested "Fill it with water, and carefully pour it into measuring cups/buckets!" (my initial thought)