What kind of box should I build?

2.5 cubic feet of airspace would be enough air to use one sub, sealed.  Putting two drivers into that box would result in a peaky, higher frequency (54 Hz as opposed to 41 Hz) enclosure.  Take into consideration the way you like to listen to music, and to the resultant impedance load on the amplifier.  Weigh these factors to come to a conclusion.

WinISD should have that sub available already in its database, but look at every parameter and compare to your spec sheet.  See the sticky topic on WinISD above for more info.

Not unusual at all.  If you have the opportunity, read through past threads in this forum for info on this subject.  This is the gist of it if you are marketing your subwoofer product to the masses:

• Show a size that is easy to find a box for.
• Show a size that will fit most vehicles.
• Show a size that will not weigh your car down extraordiarily.
• Show a size that must be doubled because the going "in" thing is to use two subs.

You see where I'm going with this?  "At what trade-off point will this sub still perform better than my competitor's sub?"

That's why there are forums like this one and the programs for box-building that educated people create...for those consumers who are not just another one of the masses.

Also, if you would have read through the topic I mentioned above, you would know that WinISD looks only at achieving the flattest response (closest to .707) as possible, no matter how much air volume it takes.  You would also have found, by reading the featured guide, that the program is designed to be a working tool...not a "recommender" of box size.

Use it to your advantage.  You know how much space you have available, the program does not.

P.S.  You will be given the opportunity to edit once you become a standard member.  Keep posting!