You don't
really need a VARIAC to control a heating mantle. VARIACs are a big, clunky, ugly, old, and outdated way to do something that the modern technology we have today can do much more efficiently, and I'm surprised at the number of people that still recommend actually using these things... I built a TRIAC "light dimmer" circit for less than $15 in parts, works fine for the purpose, and adds no extra security risks (sparks, etc). They can be built easily with a little some bit of electronics knowledge, or perhaps some research. Or even easier, bought from a hardware store (make sure you get one with a high enough wattage rating), or bought in a kit (only soldering skills required).
http://www.epanorama.net/documents/lights/lightdimmer.html
is an excellent page explaining the operation and lots of details about dimmer circuits, and has some schematics as well. The "Minimal Circuit" will work fine for a heating mantle, and you will notice that typically the other schematics have a capacitor between the "hot" and the "neutral" leads, and an inductor somewhere, which supress "buzzing" when used for dimming an incandescent bulb (explained on this site also), which isn't a problem for a heating mantle anyways.
Also note the wattage ratings on the circits on these sites. The minimal circuit uses a TRIAC/DIAC all-in-one, and the 1000W uses separate TRIAC and DIAC. You can simply replace the TRIAC/DIAC section of the "minimal circuit" for the TRIAC and DIAC in the 1000W circuit, or with any TRIAC/DIAC combination that you can get your hands on.
