How to

Melt the card in hot water
Apply to the broken area surrounding the whole cracked section.
Hold the broken object in place while the formcard hardens.
If successful the card would have stuck and the repair should be solid.
In the event the formcard does not bond to the plastic you will have to try the following tips
Score/roughen around the cracked area (this gives the formcard a better surface to stick to.
Use a hot air gun/hairdryer on the formcard to get it to go extra sticky (be careful not to overheat)
This should be enough to make the formcard stick.
Another tip would be to drill several small holes around the cracked area and make sure when squashing the formcard that it bonds together through these holes.