In the spirit of being crafty, I tend to think there are quite a few possibilities if one is so inclined.
Above mentioned was modifying the factory trigger. I think this is pretty easy by either flattening/shaping it (ie probably heat and hammering) or building it up (metal, epoxy, etc). Alternatively, one could also get a spare trigger and lobe off the lower portion and modify it to adapt a clamp-on trigger shoe.
Additionally, given that the trigger is pretty simplistic in design, it wouldn't be unreasonable to create a brand new one from barstock and some basic tooling and knowledge.