An experiment (with temperature probe) here. Summary: temperature control is non-existent, temperature doesn't hold, and even if it did get to required temperature, one wouldn't have a way to know. https://youtu.be/dSwzau2_KF8 (at 18:11).
I'd have to imagine the temperature would get much higher in the OP version (just wrap in tinfoil) than the video version (put in a glass jar filled with liquid). Second version seems too insulated.