The talk box is basically this: A sturdy metal box with a speaker inside and a hole on the top. A piece of plastic tubing fits tightly into the hole, and runs up your microphone stand. The sound comes out of the end of the tube.
The idea is this: you put the tube into your mouth, then shape the sound by moving your mouth and lips as if you were speaking. Really, nothing in the digital domain can make this sound, because the audio is being acoustically filtered, by your head!
But back in the day (when I was a poor kid living in a trailer), I used to do this:
Get an El Cheapo set of headphones (you can probably find them at a Dollar Store in the US. Break the speaker parts off of the wire thingy that goes over your head, then stick the speaker into a plastic bag. Seal it shut with a wire twistie (or use plastic wrap and seal it with a lighter). Now you've got a waterproof, small single speaker (oh yeah, you'll just need the left side for this.)
Put the speaker into your mouth - maybe between your cheek and teeth, pointing in. Plug it into your synth's headphone jack and crank up the volume. Now "talk" into a microphone and record yourself. Yee-haw! You've just built a white trash talk box for a dollar!
It won't get nearly as loud as a "real" talk box, so you'll need to be careful how you breathe. Also, the sound will be quite tinny (the same way a "reeal" talk box is). It helps to maybe EQ up the low-mids, then compress it like crazy (EQ first, then compress, so the compressor evens out the changes in tone).
Worked for me and my SK-1!
-Hoax