I have also run into the same problem. I want to practice uppercuts and head kicks more accurately on my heavy bag. Here are a few things I've found from doing some research:
1. Revgear makes a "donut" style attachment that goes around the heavy bag that creates an extended edge that you can practice uppercuts on. Problem is it's about $60 for this attachment. Kind of pricey in my opinion. You can find it on their website.
2. Century also makes a donut style attachment. Cheaper in price but does not seem as well made as the Revgear. I've used Revgear focus gloves and they're made well.
3. I saw this video of Kyokushin fighter Ryu Narujima (who is awesome by the way) working out on a heavy bag. There was an attachment on the bag. It was a square shaped portion (about the size of a person's head) that extended from the heavy bag and was attached to it by a strap. I think the strap was velcro. If I had my choice I would rather have this than the donut style attachment. It seems more conducive to training accurate uppercuts, front kick under the chin, round kick to the side of the head and hooks to the side of the head or chin. Anybody train with this or know where I can get one?