I found the exact same issue with my foam build. I built the same shins as you have. The way I fixed it was to widen the lower half by adding 1/2 spacers.Okay, disaster time:
I've printed the first shin piece. And, there's a problem... I can't get my (rather big) feet through it. So, I got out the heatgun, tried to enlarge it in a couple spots to hopefully make it fit better, And it still didn't fit. So, I took out my renovator with a cutting attachment and but off a part of the back, following one of the lines already in the model. Eventually, I got it off and after cleaning it up a bit it fits back (not perfectly, but I might be able to hide it with straps, cloth or other methods)! However, since I was busy trying to heat-form it to my foot, it's slightly different from the normal model Furthermore, Getting it off is still not easy, although it is possible now. The height is pretty good however. Since I already began printing the second one (and it's almost done) I'll have to somehow replicate all steps and hopefully get an uniform right shin piece. It'll be hard, but hopefully possible. Because of this, I'm not going to release my shin files just yet until I figure out a better solution. I'm sorry for this, but I'll do my best. A new update with photo's should be coming this week!
This might be more work for your build but, have you considered making the shin in two parts that are hinged together and then kept shut with buckles and straps? Another possible work around is have the back part only cover the calves and your ankles are exposed.