The magnetic 'fishing' ones are 60mm or more and much stronger than you need. Anything over 10Kg should do the job depending on how many you use of course.Thanks for the link.
The reason I was asking is because I have no idea what the strengths 'feel' like.
Your link is for 15kg.
Another above is 22kg.
Amazon do a 770kg, I guess that would be ott.
The ones called 'fishing magnets' have a ring bolt attached, which would be handy for pulling off.
I was thinking about use 3 or 4 on VWT4 roof. 2x3m tarp. Two poles.