Applying custom decals to plastic figures
In my scale modelling, I use Microscale Industries' Micro Set and Micro Sol which works great for clear decals.  These products increase the adhesion and slightly dissolve the decals which allows them to conform to curved surfaces better.  Once they are dry, I sand any visible decal edges very lightly with a 600 or 1200 grit sponge.  Finally, I repeatedly cover the whole model with light coats of an acrylic clear coat, building the coating up until the decal edges disappear.

I'm not sure if the Microscale chemicals will work with foil based decals, but the edge sanding and clear coat should help them stay where you put them.