The two middle mounting blocks under the Bafang battery plate have 5 mm holes for the screws going into the water bottle studs brazed to the frame. The outer blocks clamp around the frame and it seemed like a good idea to secure the plate to them, as well:
Those two blocks have a recess for an M5 tee nut:
I snipped off the prongs with hardened diagonal cutters and filed the stubs to a uniform height:
Applying a hot soldering gun to the plate melted the stubs into the block:
An M5 screw with a wingnut atop a big washer kept the tee nut properly aligned while pulling it into the melty plastic; I was pleasantly surprised at the lack of drama.
A ring of JB Plastic Bonder urethane adhesive glued the nut to the block:
The adhesive starts out runny and flows under the nut, so there’s more surface in play than meets the eye.
Clamping the partially cured goo to the frame atop a layer of waxed paper squashes any protruding adhesive lumps flat and prevents them from marring the tube’s paint:
Anchoring the battery to the bike at four spots makes it utterly immovable, which seems like a good way to ensure longevity: