i can give you ideas on how to go about making one. you need to go into your trunk...or wherever you are goin to put this box, and make a stinsel.. then you will need to make sure you cut out the stinsels. also making sure that you leave enough room in the interior of the box to meet your subwoofers specifications, which can be found on the box the sub came with. after that you can go ahead and start cutting out the pieces of wood using your stinsels. after each peice make sure that it still fits in the area you wanted it too. next you will need to cut out your port...if you are wanting to port it... do that step while you are cutting out the holes for the subs. then after all the peices are checked for fitting, you can go ahead and use wood glue and a nail gun...give it about 45 minutes to dry, and make sure it fits once more....then go ahead and fur it with the color of your choice.
these are kinda ruff instructions, but you should be able to get it from this.