The drill screams!
Time framesets and seatpost internal battery adapter sets come with a junction part that's fixed below the stem with rivets,
but I'd guess most shops don't own a riveter.
（You could just buy one for this, I suppose）
Since it's a square-section stem and pretty large diameter,
it might be better than securing it with the standard rubber band.
There's a hole for the riveter on the back of the stem,
but it didn't pass through smoothly, so I lightly cleaned it out with a drill.
Done.
Even with just one point of attachment, it doesn't move an inch—
probably helped by the rubber sheet layer sandwiched in between.
The included rivets looked like they'd rust if the black surface layer chipped off,
so I used a different rivet with the same shaft diameter.
↑Looks like this
Time framesets and seatpost internal battery adapter sets come with a junction part that's fixed below the stem with rivets,
but I'd guess most shops don't own a riveter.
（You could just buy one for this, I suppose）
Since it's a square-section stem and pretty large diameter,
it might be better than securing it with the standard rubber band.
There's a hole for the riveter on the back of the stem,
but it didn't pass through smoothly, so I lightly cleaned it out with a drill.
Done.
Even with just one point of attachment, it doesn't move an inch—
probably helped by the rubber sheet layer sandwiched in between.
The included rivets looked like they'd rust if the black surface layer chipped off,
so I used a different rivet with the same shaft diameter.
↑Looks like this