IMHO, the best buys on the market are the red fiberglass jungle issue stocks. They are slimmer and feel much better than the wooden issue stocks and are extremely rugged.
You can find them for under $10 and if you don't like the color, you can always paint it. My buddy did one in tiger stripe and it came out really sharp looking.
I also found that by putting a ventilated handguard on it and drilling a vertical row of 1/4 inch diameter holes through the bayonnet channel, it keeps the barrel MUCH cooler during rapid or prolonged fire. I spaced the holes on 7/16 centers and ran them the entire length of the channel. I was surprised that the extra ventilation made so much difference.