You're right and I know that 90°C is not a so much temp. But in a so small box, it's to much IMHO, just because there no air, no hole that would allow heat exchange. I will do some test with the 100mA charging rate, and give you back the temp of the 7805 in that case.
By the way the 7805 will get an 12V on its input, I suppose the temp will be around the 120°C, I can use it in that way. So from 100mA will do the job I hope.