well could use a heat gun that works or could drop your stuff in boiling water let it heat up and then bend it. But what ever you do its going to need heat so watch your self. Im guessing you used some kind of plastic?
Mine is the same thickness Sintra and I used a large pot of boiling water, works great and no worry on fumes from a heat gun. I've also heard that you should get an old used pot, as large as you can find, that you wont bne using for food. Boiling the Sintra can leave toxic residue in the pot.