a device in a bag in a hot room will have terrible heat dispersion. CPU generates heat, so does an LCD screen ( but maybe not an e-ink one? ). that heat has to escape somehow.
Run a PC in a plastic bag at it will quickly become unstable due to heat build up
the good news is that most devices will recover once they cool off *, so trial and error may show that you are ok for 20 - 30 - 40? mins reading in sauna so long as you then emerge and pop it in the fridge for a bit
* the exception to that is an exploding overheated lithium battery