Now, not something I would recommend but the holes were there already so thought I'd give it a go.
Roof rack is on.
Added some aluminium spreader plates I made up and bonded onto the inside of the sides of the poptop.
Added more foam pads where the brackets rest on, so any load is spread pretty evenly over a large area.
It is very secure... I was very unsure about it but with it on I'm actually quite impressed.
Never going to hold much weight, maybe a couple of surfboards... the roof box might be a bit much.
I've new struts here to adjust to take the bit more weight, but actually the current struts hold it fine.
Test drove last night and yes there is a fair bit of wind noise that kicks in around 60mph. But nothing terrible, music will drown it out rightly.
And I really don't want to go much faster anyway. at 55mph you wouldn't notice.
I will always now have 2 cable ties as well as the straps keeping the roof down, just to be extra careful. But if there is a better secure way I'm all ears.

Thanks to my Dad for helping.

The spreader plates. Note the nuts were just for holding it while it bonded, the actual brackets have proper stainless lock nuts.