I did a bit of digging for you and found this post.
So basically your box can be a few different sizes for that sub. The manual should give you the sizes unless you don't have it anymore.
Sealed Enclosure 0.5 CuFt F3 = 45HZ
Sealed Enclosure 1.0 CuFt F3 = 41HZ
Sealed Enclosure 1.5 CuFt F3 = 40HZ
Ported Enclosure 1.75 CuFt F3 = 30HZ
As you read that post, you will see that it was recommended to go with the 1.5cu ft box SEALED.
So basically in this case I would build your box to have outside dimensions of a total of 1.5 cu ft.
as an example it could be
16" wide x 15" high x 11" deep that would give you 1.52 cuft
If you went with 10.5" deep that would give you 1.45 cuft ( approx), so you would be close to it, and still within the parameters for that sub
You have to make sure that you have enough height and width for the 12 face to fit on and enough depth so you have room behind the sub.
That is just an idea for a box dimension. If you have more height, then you can go higher, if you have more width then you can go wider, that will allow you to make a shallower box, so it leaves you with more trunk space.
I hope that helps