I have fitted arm rests to my Caterham-supplied S3 half-doors. It is a little fiddly, but they do make huge difference to driver comfort and also help fix the half-door to the car. Mine have poppers front and rear, so yes I think you will need to drill and fix poppers on your scuttle. As mentioned above, tape the doors firmly in place first, and mark the holes with a centre punch. I was able to push a sharp screwdriver through the doors to create the popper hole, so something similar could be used to align the door first.