In reply to Dave the Rave:
Shouldn't really need J-bars, unless you have other stuff on the roof - I use uprights when I can't just put my boat direct on the roof bars, or taking >1 boat. Anyway, the principle is the same and well explained here (among other places):
https://palmequipmenteurope.com/blog/2021/12/03/how-to-transport-your-kayak...
You really don't need to over-tighten the straps, or you could damage your boat, especially if it's hot and the plastic softens - I fully endorse straps to the front/rear grab handles as back-ups.
You shouldn't need to pass anything through the scupper plugs, but can be a great solution to make sure the boat can't slip out of the straps. Also good for putting a lock through.
Many years ago, I was a victim of the "lemon pip" principle, which was a combination of the shape of the boat, the width between the bars on that car (fixed mounting points) and getting stuck for 2 hours in a traffic jam on the M4 on a stinking hot summer day. In my case, I was the reverse of the 2nd pic in the link above - I'd purposely hooked the back of the cockpit behind the front roof bar so it couldn't go forwards under any amount of braking and I'd thought the widest part was just forward of the rear bar, but a softened boat and vibration on the road must have conspired to loosen the straps just enough that it shot out backwards on the M4! Fortunately, no other traffic around, I saw it go and, by the time I'd got to the hard shoulder, it slid to a gentle stop just behind the car! Boat was fine and duely returned to the roof in a modified fashion!
My current boat has a flat bottom, so sits upright on a couple of foam pads and no issues at all with strapping it on.