When I measure the difference between the carriage and bed from front to back along the Y axis, the difference is 0.14mm. I'd say that's good enough, especially when you consider it was 2.75mm difference before. I used rsilvers' angle of 1.5 degrees, which turned out to be perfect on my machine.
Here are some more pictures of the parts. There is a front and back part. The back part is the wedge on the left, which has a slight angle so you can push it in until it firmly holds the front part in position:
Here is a view showing the front piece in place. I only have two zip ties on at the moment, which turns out to be really strong because of the positive seating of the bearings in the curved holder:
And finally, here is a view from the back, so you can see the wedge in place:
The front pieces each take about 40 minutes to print, and the wedge a lot less time (I don't remember how long). I have a few more changes I want to make to the wedge as it's a little too tight right now. But at least this solves my Y sag, and may solve it for others as well.
You can find it here: http://www.thingiverse.com/thing:99364
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