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.