Question on fiberglass

Use the calculator on the left side of this window and that will give you the dimensions you need.  If you have curves don't freak, just use that formula from calculus 3 and use the function of the derivative of the curvature of the plane and then take the intergal of that function and add it to the area of the plane.  Not really, actually sealed boxes are rather forgiving w/ volume.  So take the best guess  you can and use that.  If it's too small then add some polyfill (pillow stuffing) and if it's to big add some blocks of wood (glue them down to a flat part of the box).  And don't use termnal cups.  It's hard to seal those things up.  Unless you have a flat surface then you might concider it but I would just do a small hole and pull the wire through.  Make sure you put some silicon caulk around that to seal it.  Show us some pics of the project when you start/finish.