Ok, sorry for the delay but I took some measurements. All mag measurements were taken on the top side of the mag catch side cut out.
HK Steel .996"
Just to check, I also measured the inside width of the mag well.
Rear, near the mag catch 1.074"
Looks like I could compress the mag well a tiny bit if I felt so inclined. I may try it one day but with only roughly .03" to work with it may not be worth it.
It is amazing how much rattle/play the .005" difference between the MKE and HK mag causes.