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Okay, okay, something I actually feel confident voicing my opinion on.
Do you have basic roof bar mounts? The car I mount mine on is unconditional to say the least, so, get a cheap cross bar kit
Use pipe foam insulation rods, it's cheap as fuck, will compress under stress but will hold its shape, and then just ..
Ratchet straps, Ratchet straps, Ratchet straps
(putting some more pipe insulation foam around areas where the straps might meet the car and then running duct tape around it reduces damage)