Same issue and fix for me as well (so far)- arch support is a life changer.
If you can't get a hold of the specialized brand supports, Superfeet carbon are a good alternative, though designed for flatter feet, it provided my monstrosity of an arch with adequate support to rid of any hot spots or discomfort I'd traditionally get cresting over the 3-4 hour mark.
Couple worthwhile considerations beyond arch support;
1. Pedal washers: uncompensated forefoot varus positioning, which is very common, will ultimately place more pressure down on the outer edge of the foot. A quick trick to compensate, though unlikely to 'rid' of the problem completely, is by widening your stance with a washer or two.
2. Saddle height: too high? Maybe you're extending the hot spot side to compensate for the additional reach.