I have drilled a hole 14mm-9/16" smaller than the round part of the boot in the A pillar. I made this tool to flare the hole. The slot fits over the edge of the hole like this.
Then you pull up a little bit at a time and work your way around the hole. The T handle works well making sure the slot is always bottomed out while pulling on it. I used the small mallet to help it around as the flare got over about 45*. Could also hit against the underside of handle to pull the hole level if it sank in at all. Once nearly there I tapped through the hammer which was the size I wanted in the end. Helped round the hole a bit more too.
It left a 7mm-1/4" flare which will stop the corrugated part of the boot getting caught as it goes in and out while using the door. Once the flare was finished I was able to pass a holesaw right through and drill out the other side of the A pillar to get the wires and plugs through. So the hole at the start was 29mm-1.1/8". The 10mm-3/8" slot opened up the hole to 43mm-1.11/16", 14mm bigger. I had done a test hole first to see what I would end up with. But the slot needs to be half the depth you want the diameter to increase by, plus about 2.5 times the material thickness.
With the plugs all the way through the A pillar and the wide part of the boot inside it, you can push and pull the boot in and out even on the angles it will see with the door opening and closing without it catching. Now the door side I don't want to move so I want the final hole size to be the small diameter of the corrugated grooves. I am still going to make a small flare so the edge of the hole doesn't cut through the boot over time. Notice the angled cut along with the other one on top.
The angled cut is so I can start the flare and clear the hinge and the edge of the door. I switch to the other slot once about halfway done flaring. So this hole was 25mm-1" to start with it ended up at 33mm-1.5/16", 8mm-5/16" bigger. This was with a 5mm-3/16" slot and with a 4mm-5/32" deep flare.