It sounds to me like this will not work if I actually power down my device before the attacker gets their hands on it, is that correct? (I tend to power down my phone before going through TSA lines, for example)
Fun fact: This is why during raids against cyber criminals reports claim they often dive for their computer to try and turn it off before being restrained. Police can do the same thing with liquid nitrogen and a desktop machine.
I'd bet that most people that think they are in this risk category do not have strong enough security practices to prevent data from being cracked by other means.
Thermite, even the more explosive copper-based variant, is pretty poor at destroying disk platters, sadly. Explosives or cutting equipment seem to be better, although the magnitude of the engineering challenge obviously increases.
Here's an alternate idea: rather than messing around with chemical explosives, how about filling the computer with poisonous snakes?