$300 will be a stretch if you can't build the box yourself (lack of tools).
If it was me, I would go with a 3.5cu ft enclosure from the site I sent you above. At that point you just need to buy some wood glue and maybe some clamps to assemble the box. Then I would buy some primer and paint (spray cans work great, just go online and learn how to properly use spray paint and what techniques to follow). Probably looking at $40 or so for all this.
This does not leave you too much for a woofer and amp. You could get something like this:
And put it in a smaller enclosure (say 2.25 to 3ft) and then add an amp like this:
The trick will be getting the 12" driver to fit the enclosure (pre-cut) and you would want to contact Erich at the DIY Sound Group to see what he can do for you.
If you want to put a little more money into this, you could get a woofer like this one for a 3.5 cu ft enclosure from the above:
This will give you more output and extension, but it clearly costs more and you would need a beefier amp to really benefit. I would go with an external amp like an Inuke 1000 that can be found for around $175 shipped. But you would need around $450 for the above.
Ported subs are a great way to go, but if you don't have any tools, will be more difficult to build. Ported subs are typically larger as well.Edited by ack_bk - 2/1/13 at 1:18pm