Just to be clear, you canNOT have DeWalt 20V Max tool compatibility with B&D 20V Max batteries just by grinding some plastic. The tools are electrically as well as physically incompatible.
It’s actually more feasible to use B&D 20V Max batteries with the older DeWalt 18V tools. This is what I run myself, allowing me to use my DeWalt 18V tools and B&D 20V Max tools all with the same B&D 20v Max batteries. There’s a guy on eBay who sells an adapter for this. Just beware, his instructions were (are?) wrong from an electrical standpoint.