The 103 is a heavy horn and I've always struggled with the left hand.
The strap transfers the weight to the hand instead of the pinky. I've even completely removed the adjustable finger hook.
To attach it to the 103, i position it much lower and then adjust the strap to the correct size. It's a big loop, which is comfortable to slide in and out of.
Also, I use it upside down, to ensure my hand just touches the bare strap webbing instead of the soft part of the velcro.
The only downside is the velcro catching the case lining, but that's unavoidable with any velcro.