I can't speak to using a K28 insert with a B30, but I did have a similar issue with 7RP that seems to be fixed with MT. I bought a 7RP for my ostrich B30, and was thrilled with it--great quality and a perfect fit. For my BF B30, however, I noticed the same thing you have--the corners were pushing on my bag, and I could see bulges from them on the outside of my bag. I'm not sure if it's due to slight variance in the bag sizes or to BF being a much softer leather. Regardless, I was concerned about my BF and didn't want to test out another 7RP given the cost. I just got a MT, and am happy so far. Admittedly the fit doesn't seem quite as precise and it doesn't have as many functional pockets, but there are definitely no corner bulges to be concerned about with the MT. On balance, I definitely prefer it for my BF, and will continue to use my 7RP with my ostrich.