I use plastic wire ties threaded through the plastic air vents.
I place one vertically and tighten it down to a small oval.
Then, I take one (or two tied together) and place it horizontally through the first one. The scanner has to fit down into the horizontal holder, so don't make it too small. The secret is snug, but loose enough to remove the scanner. The belt clip and friction helps to hold the radio in place.
Adjust the height of the vertical tie to allow for the scanner's LCD screen to be seen, as you require.
Another trick is to use silicone.
Remove the belt clip if necessary and place a nice glob of G.E. 2 brand clear silicone on the dash(from a home improvement store for a few bucks.....the size that'll fit in the glove box and doesn't require any tools). That'll hold it in place nicely.