Most of the fabric bikini tops have plastic flaps that tuck into the door surrounds, but straps also, should you want to just strap to the roll bar above the door. The strap to the roll bar method leaves the top looser, and more prone to flapping on the highway.
The thing about bikini tops vs. a spiderwebshade or alien shade is they need a windshield header to work, and you have to remove that to put on the soft or hard top. A bit of a PITA, unless you plan to just run with the bikini for months at a time. The spidey or alien can stay on all the time, full top or not.