If you don't have hex dumbbells, you can squeeze a foam pad or something similar between both dumbbells. The key during the squeeze press is to squeeze inward as hard as possible; this is more important than the weight used.
Lift them up slowly to maximize the squeeze. Then move on to regular flyes, getting a good stretch in the bottom position. Finish with high-position flyes in which you bring the dumbbells down above your shoulders and lift them back up above your chest.