My version of the Handi-Quilter HQ Sixteen grip control caps requires some assembly:

Getting the OEM caps off the handlebars required carefully applying torque through a strap wrench, but they eventually came free:

I don’t know what the unused screw hole between the two gnarly holes was for; perhaps they discovered one hole was inadequate.
The alert reader will note the two screw holes are not the same distance from the end of the tube, which required rebuilding the plug model to match:

Which is why I didn’t glue the plug into the cap before I got the OEM caps off.
Redrill the tube holes to 3 mm, file the burrs from both the OEM and my drilling, smooth the edges, and the plug fit perfectly. Then I seated the M3 square nuts behind those hole and, after installing the new plugs in the handlebars, glued the caps in place with a simple fixture to ensure the front faced forward:

The clamp gently compresses the foam enough to hold the flats against the bench block while the JB Plastic Bonder cures.
After verifying all the buttons worked, I glued the faceplates to the cap bodies:

The tape held the faceplate in place while I snugged the clamps.
Modulo my weak graphic design skills, the caps look pretty good:

And, after a bit of wiring yet to be described, the buttons do exactly what their legends suggest:

The white sheet with feeble graphics can be peeled off, so I have another chance to tart it up.
The overall idea was to replace the failing Start/Stop switch while duplicating that switch on both caps. While I was at it, I also duplicated the Needle Up/Down button, because who wants asymmetric caps?
Mary is assembling another quilt and the new switches will get plenty of action …