Custom Enclosure

OK im a stupid newbie when it comes to building custom enclosers. I have been reading about how to make them to fit the specs of my subs but its not working to well.

This is what i have 2 Alpine 10's SWR-1041D. The specs for Recommended Sealed Box Volume is 0.5~0.8f^3. What that number means. I have no clue.

So basicly this is going into my 300zx. Im using 5/8's MDF board. Im tring to make this box very small in hight but wanting to make up for it in length.

Pretty much what im asking for is how would i figure this out?? Please respond like your talking to a 8th grader cause im very new. Thanks for your help.

Your speakers need 0.5-0.8 cubic feet of air space, each, that means you need to build your box between 1 cubic foot, to 1.6, (since its going in a 300zx, probably  1cubic foot total for the box......find out what the depth of your speakers are, and go from there, measure the space in your car where you want to put it, or can put it...then go to the Calculators section on this site, and use the calculators to figure out what size your box needs to be to get 1 cubic foot.....let me know if that helps..

Ok so let me know if this is right.. I messured the area im going to put the box

42" width

17" depth

18" height

SO my box settings are as follows  20" width, 10" height, 12" depth, with a thickness of .625 (5/8's mpf).

That would = 1.02064 Interior Volume...

Does that sound about right??