Here are the best and cheapest options I've found for voltage control so far:
A light dimmer, very cheap. It seems to work decently but if you weld too fast or too large rings it will burn out very quickly. Unplug it before you weld high resistance rings. You'll also need to be able to do your own wiring.
$6
http://www.canadiant...r.jsp?locale=en
This is a lamp dimmer, it's only two prong so you need to cut off a bit of plastic to plug your welder into it. Unplug it before you weld high resistance rings. It only has half the capacity of the light dimmer(burns out very easily) but it's already wired up.
$11
http://www.canadiant...r.jsp?locale=en
A 5 amp variac, about the same capability as the light dimmer but it has inexpensive and replaceable fuses that blow(you might want a couple). It is also plug and play and has a built in voltmeter. Unplug it before you weld high resistance rings.
$50
http://www.circuitsp...itml/icOid/7674
A 20 amp variac, this has a higher amperage capacity than the welder itself, it has replaceable fuses but I doubt you'll ever blow one. It's ready to go, has a built in voltmeter, two outlets and comes with a coupon for a free electrical multitool/tester of your choice. Seems like a good deal to me, I'd get one if I didn't already own a variac.
$104
http://www.circuitsp...itml/icOid/7673