Cut stripes or shapes out of card and using blue tak, attach them to the area the mottle is to be applied and use a spraycan. A quick pass or two with the spraycan held vertically over the area to recieve the paint will give a good result. Light mist coats are best till the pattern builds up nicely.
By raising the mask slightly, you will get a nice feathered edge.
You can cut various patterns and apply the same technique as you go.
Mike’s idea should do the trick. Works very much like the silly putty idea.
It also helps if you can spray away from the edge of the mask instead of towards it (aka under it). This is a little harder to manage with a spray can on a tank since the masks are so close together.
I saw this japanese article about how a particular japanese modeller would actually paint the “soft” edge camo on old biplanes using a tiny pointed brush, with tiny crisscrossing lines. Not really very “accurate”, but interesting nonetheless.