For those unfamiliar with the blowfish technique, please see the linked "Blowfish Tearaways". But consider the blowfish. Often the recipient can see it coming, which impairs the fun. The window might be dirty, or perhaps hard to access (like the windshield of a moving car). The window might be on
a second floor bedroom and so hard to reach. Or one might consider a different face or other body part more suitable blowfish material than one's own god-given equipment.
BUNGCO believes like you believe. The heart of the Delayed Flipup Blowfish apparatus is a small spring akin to that found in a clothespin, together with an ice cube tray. Bend the spring then add water and freeze into a cube. Attach your blowfish image (easily made with a copier, or a wide assortment available for purchase) to a piece of substantial cardboard (sold separately).
Now one prong of the DFB spring is attached to the substrate - maybe a magnet on the car, a piece of tape outside the window or a long stick in the case of second floor windows. The other prong of the spring is attached to your blowfish image. Place it and sneak away, tittering. When the ice melts, the spring will release, swinging the image up and around and slapping it into the window! YOW! And best of all, after he or she gets over their surprise, the recipient of the DFB can reclaim the apparatus and repurpose it for their own DFB high jinks.
Suitable for use in temperatures above freezing.