I also prefer the hot water method.. You just need yto have a container that is big enough.. I also haved used a heat gun with excellent results. You just have to be careful not to melt the sintra..
We've never used boiling water to shape sintra so we can't say how good it is. :facepalm
We have used a heat gun and have had excellent results.
However, if you decide to use a heat gun, be sure to heat the back side of the sintra and not the side you're going to see. The heat gun gives the sintra a weird texture.
Hope this helps.
Or you could get a Heat Gun from the local Walmart (where I got mine) for about $20-$25 in the hardware/paints area (it's cheaper than the hardware store...believe me!). Just remember to use it slowly and one step at the time. Do not rush on it, it could be a really "burning" pain if you're not carefull, not talking about the money invested in the sintra and the work to cut it in shape.