About fastening to the rack- I don't understand people who will spend either:
a) a lot of time and a good amount of money
or
b) money, possibly thousands of dollars
on a kayak and not buy 'proper' cradles for the roof rack, and 'proper' tie-down straps with padded cam buckles for fastening the boat to the rack.
A pair of 'proper' kayak straps costs under $25.

Here's the understanding:
Home made purpose built look like old bits of driftwood with padding that sit on the bars,come off just by lifting them off stow in the car and cost the time it took to make them.(very little)
Here's the reason:
They are an extension of what you just did building the boat,better at what they do,by far,and completely at odds with the rest of the world who,well,just don't get it but jolly well should,by now!
It's good to be different.
Here's the corollary
I don't understand people who buy cradles that don't work as well as they could are stuck on the bars full time because they're a pain to get off and cost an absolute fortune!
I'm just having fun,as usual but....I hope you get it because the world is full of people who take it far too seriously and have lost sight of what is actually important....Getting to the water and going for a paddle,in this case,for very little cost and with a considerable amount of thought and awareness.