I have several composite sea kayaks including an enormous 7m sea double which requires a bit of thought when on the roof rack. I’ve also a small flotilla of white water and surf kayaks.
I use straight forward uprights and the boats go on their sides, hull against the upright with the gunwhale in foam on the roof bars. Straps go through the eye at the top of the upright and then round the roof bars. Another boat can go on the other side or against the deck of boat one
I have found that the uprights are the most versatile and will cater for many types of boat, plus also good for ladders and timber for diy etc, so they are multi use.
J bars are good but not as versatile, and also slightly more difficult to load as you need to get the boat a little higher to get it in the cradle. Difficult if you or partner are short. Also less good for said ladders and diy materials. A free pair would be fine though!
Sitting composite or plastic boats in cradles is ok but it’s possible to over tighten if not careful. On a hot sunny day it’s easy to reshape plastic boats and some epoxy based composites, especially dark coloured ones.
I have also had boats straight on to bars with only foam, fine as a stop gap and if it’s your own boat. Same problem with overtightened straps and hull deformation
Current uprights were about 25 quid, aero ones probably a bit more.
Hi viz vest looped through the rear overhanging stern grabloop, length of cord from bow toggle to bumper.
As matt says, try and get the roof bars as far apart as possible, although many modern cars are pretty woeful in this regard.
If the boats are flat on bars, try and get the bulkheads within the boat on top the bars, it’ll stop the hull deforming