I usually shim up the trigger by putting a small piece of credit card on the trigger stirrup.
If you are holding your trigger stirrup and trigger in hand.
The point where the stirrup begins to curve is where I ad my shim. Ideally, the card goes right before the stirrup bends, and touches the inside of the front of the frame, which takes up some of the slack. However, it would be best to get an adjustable stirrup if possible.