This product is a bit pricey and not shoulder specific, but does offer a solution for not having a freezer - they offer a bottle that keeps the cold pack frozen for up to 6 hours. http://www.gomoji.com/products/moji_to_go/2.php

they only offer knee and back specific products right now, but they are pretty cool (I've tried on the knee model - its comfy).

My other thought was if you are driving you could possibly strap the ice pack to the car seat rather than trying to put it on your body and still get some decent icing in, as long as you don't sit slumped.