mattv2099 has finally revealed how he makes those Bumpinator stocks, which are a sort of ghetto, redneck-engineering versions of the Slide-Fire and Bumpski stocks.
Most of the work can be done with a Dremel tool. I'm not sure about the square tube, but the other hardware you can find at your local ACE or Home Depot. This one seems to be for the AK/Converted Saiga series of rifles and shotguns, but with a little ingenuity, could be applied to SKS and 10/22 rifles, as you can see in mattv2099's other Bumpinator videos. EDIT: Nevermind. I forgot that the pistol grip is attached to the butt, and not the part of the stock that the action drops into.
Here is Br0dBaNd's home-made AR-15 bump-fire stock. Because of the way the grip attaches to an AR, you have to do it differently.
(For educational purposes only, obviously, Suburban's Domain not responsible for any damages to persons, or property, bail, lawyer fees, or anything else, and I will not make one for you either)
EDIT, 2/27/2013: Matt added a new video on how he did the 10/22 Bumpinator.