Harness for 3 legged chi

Hello fellow chi enthusiasts!

I have been struggling to find a harness that will reliably stay put on my 3 legged chi.

He gets spooked easily and slips out of harnesses quickly. He can move pretty fast considering 25% less legs.

The missing leg is a front leg. He is deer head with a thick neck, healthy weight but also getting a bit old.

TIA