Many rifle safeties use a ball under a small leaf or wire loop spring. The ball falls into a small hole on the plate that moves underneath. Hardware stores often have a selection of single steel ball bearings. If you can find a place for the ball and spring in the side of the frame, you could drill a smaller hole in the trigger plate for it to land in. Start the trigger plate hole small.
You would need to set up your trigger pull first. If you think it still needs a detent, you would set up the detent next and then mark the plate at the tripped position through the detent hole, then drill it. If the detent could fit under the grips, it would be extra fancy and harmless.