300 dollars??? doable if you're happy with OLD equipment.
make it 5 and we can work with ya //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
-you could grab a set of high quality subs from the FS section on here for 200 or less.
-a good amp for 200 or less.
-40 dollars for the box (20 for 3/4 MDF, 6 for screws, 2 for glue, 12 for spray primer and paint...)
-60 for wiring (30 for a walmart kit, 20 for the walmart scosche LOC, 10 for a spool of 14 guage speaker wire)
edit: if you don't have clamps, a drill and a router or jigsaw (or hand saw if you wanted to tough it out) it'll be more difficult than necessary and the box will suffer quality wise (split MDF ends from not predrilling, holding the box by hand while predrilling holes = uneven box, etc)