I work for municipality in Washington and we glue beacons on all our city vehicles including tractors with fiberglass/non-metal roofs. The beacons feet have a much smaller footprint than what you have and we experience very few failures typically only due to local automated car washes. The trick is surface prep but basic. Clean oxidized surface and clean pads on beacon with alcohol. I use PartsMaster brand Adhesive "Flex-Master UV". A small blob on each 3/4" round foot on our beacons and once cured overnight the beacon is there for life. I have done more damage trying to remove the beacons years later so I usually leave them on roof when vehicle is sold