Ahh. So many ppl like those but they are so rigid I don’t get it.
So the problem with heating it up is it will take more than some hot water, so you need a heat gun.
Now the problem with the gun is, if you focus too much in any one spot the print walls will start to collapse. You can start real slowly.
You are better off investing in a shell that has some flexibility. Now obviously no faceshell will be contoured to you exact head shape but there are shells that are flex to allow different head shapes to wear it.