The pill looks like it is designed for a 16mm MCPCB?, the triple is 20mm.
You can either fill the recess with some extra metal, making sure of a level surface or file the edge down flat depending on your build.
Don’t you need a spacer now?
There are ready made spacers that should fit into the pill as is.
I wouldn’t reduce the MCPCB diameter to fit the pill.
You will need a centre hole drilled for the wires too.

Tricky with the retaining ring and those 7135 chips, the pill is designed for flat drivers. Might be possible to file the inside of the ring to shape to clear the components or just solder the driver in while lightly pressing it into the pill so it stays even, probably need a filler in there around the edge like some solid copper wire etc.