What you are searching for is known as the Garthwaite trigger. Unfortunately, Mr Garthwaite passed away a couple of years ago and his triggers went with him. It looks like Girsan is having them made for their clones...I bet they make them in-house, and I'm unsure of their compatibility across other Hi Power/SA-35 clones.
Even with my impressive Google skills, there isn't much out there about flat triggers for a Hi Power.
BH Spring Solutions makes some wider triggers, but still curved.
You might want to send an email to Girsan/EAA to see if they will sell you a trigger.