absolutely no idea, I’m terrible at maths. You’d probably have to start with the heat capacity per volume of the material you want to use, then apply some funky equation to calculate total heat input, minus any losses, and get to a junction temperature of the LED.
To be honest, you’d be better off either building one from scratch or buying something that’s more easily moddable. I’ve done both and building from scratch is far far easier, if sometimes a little uglier.
Modded
Scratch built
The scratch built one is used pretty often, usually when I’m on the car (I have a Ford, so that’s quite a lot), and it’s held up fine to being dropped multiple times off various bits of the car. I recently swapped the LED to a NW XM-L which is much more pleasant to use. The modded one only gets used on camping trips and summer evening BBQs, but it does do that job very nicely. It also has a 3 bay parallel wired 18650 holder in the base, so I can use my 20+ salvaged li-ions in it if the power goes out.