the insulated container idea is probably the most realistic option--but you would have to be extremely prudent and logical and do some tests with an actual setup, with actual pannier (darker coloured ones get a lot hotter than lighter coloured ones, I know, I have both) and a thermometer to see what temps you are REALLY going to have in a X container in Y temps with Z cold packs etc for a given amount of time in the sun and at 70 or 100 degrees (give or take a abunch more due to sun on pannier surfaces)
and getting reliable , no nonsense info about what temps a given medication is supposed to be kept at--I mean, this is your health, and i wouldnt mess around with "oh this is probably ok" with crucial medication, especially insulin.
hope you can get the right information and be realistic of how a given insulation solution will really really work in real life.