This thing saved my life. Okay, not literally. But it really did help my practice out a lot. The Hugger Mugger Foam Wedge is a simple but highly effective device that has lots of handy uses. It costs about $15, and is more than worth it.
I use this prop primarily for one thing, although I have started to find other uses for it over the past weeks. The main reason I got the wedge is to take some of the intense angle off in Urdvha Dhanurasana. Especially if you have a tight back and your back bend is not so deep, it’s a good idea to use the wedge to protect your wrist so that the wrist joint doesn’t hyper-extend when you come into the pose. Also, the angle of the wedge allows you to really work the back bend deeper than you might normally be able to because the wedge virtually eliminates any risk of your hands sliding away.
Another good time to use the wedge is in Down-Dog. You can put the wedge under the mat and slope it away from you so as to take off the angle/flex in the wrist joint. I’ve never had a great time with this, actually, but some people like it and find that it helps to relieve pressure in the wrist, not only in Down-Dog, but also in Chaturanga and Up-Dog.
Lately, I’ve also found another use for the wedge. I sometimes put it behind me, under the mat, and lean my hand into it when I’m in a pose like Ardha Matsyendrasana. This allows me to twist deeper and to keep my spine upright (i.e. the shoulders over the hips).
OVERALL: B+/A-
This is a really good product, overall, and I only have two complaints about it. First, it’s not long enough. If you’re using this wedge for, say, Urdvha Dhanurasana, you need to have your hands separated shoulder width. If you have average width shoulders, and your hands are spread out on the wedge, there is no way your entire hand is going to stay on it. I probably have narrower shoulders than most, and my fingers are dangling off the edge of when I use the wedge. Hopefully, Hugger Mugger releases another wedge that is about 2-4 inches longer to address this issue.
Also the wedge is just a little too squishy for my tastes. I need something a little firmer underneath, especially if I’m doing something like Urdvha Dhanurasana where a lot of weight is coming down into the wedge. Hugger Mugger does have a new cork wedge, which might address this second issue.
Om Shanti: A Yoga Blog 
Wrist pain is a problem for me, as well in Upward-facing Bow, as well as transitioning (jumping, usually) from Downward-facing Dog to Standing Forward Bend.
May I ask how wide the Hugger Mugger wedge you purchased is? I infer from what you say above that it is narrower than your yoga mat. FWIW, I couldn’t tell from their website how wide the cork ones they now carry are. I may give them a call on Monday.
Have you tried Joint Protection Products’ Wrist Assured Gloves?
http://www.wristprotectiongloves.com. After a couple of falls several months ago (hurting both hand and wrist), and continuing although slowly lessening pain, I’ve given these a serious try. They sort of cut between my index and middle fingers, but at end of class, no soreness!