I'd worry more about placement of the uSD card under the hottest parts of the board, where there's 0 air circulation in the original rpi case.
"The higher the temperature that the NAND flash experiences, the greater the acceleration of charge de-trapping mechanisms that could lead to random data bit failures. NAND endurance is also impacted since endurance has an inverse relationship to data retention, and the rate of wear-out of NAND cells is affected by temperature at the time of programming and erasing NAND."
"The higher the temperature that the NAND flash experiences, the greater the acceleration of charge de-trapping mechanisms that could lead to random data bit failures. NAND endurance is also impacted since endurance has an inverse relationship to data retention, and the rate of wear-out of NAND cells is affected by temperature at the time of programming and erasing NAND."
https://www.eeweb.com/profile/eli-tiomkin/articles/industria...
This effect is very significant. It's like the acceleration being 250x higher between 25°C and 70°C.