On a slight uphill incline my driver-side door holder won't hold door open. Searched forums and found nothing for door holder, lots of cup holder hits.

Is it possible to adjust the driver-side door holder to prevent closing on an uphill incline?

I've been stuffing in a rolled up T-shirt to hold the door open but that's too rice.

You need to replace the door holder assembly. It's not hard to do, and isn't too expensive. I did this for both front doors on my legacy. They wouldn't hold the door open for anything.