I never tried the oven method, mighta if I had an oven in my shop... can't fit the headlight in the microwave
I got it pretty down to kinda easy with a heat gun... the trick is patience & keep heating it all around until the whole perimeter of the seem is nicely warmed up through & through,
I just warm the whole bottom edge corner to corner back & forth moving the heat gun at a good speed back & forth & try to heat it all even, then flip it over & do the other side, but only half as much time, cause you don't want the other side to cool too much, you then flip it over again & bring that side up in temperature again, flip & repeat, trying to bring up to, & maintain heat all around evenly...
when you first start to heat it your just heating the outside, but as you continue to maintain these temperatures by keep passing over them the inner seem starts to soak up the heat, after doing this for a steady 3,4 or 5 minutes, grab it at the inner big corner & outer & just pull it apart, after you get the first third apart the rest is easier.
maybe the oven might be easier cause it does all the heating for you, but if an oven is not available it's not to difficult with a heatgun either, no gloves needed doing it this way too