I use a dashboard mounted cup holder which I put my handheld scanner in, so that the standard antenna is above the dashboard. I have had years of good results using this method.
When my daughter passed her foundation exam, I also brought a micro mag, so she could operate her handheld from the back of the car & I am now on my

mico mag antenna. These are as cheap to buy as a longer antenna for the handheld, but they work brilliantly for what they are.
The problem with a scanner is we use it to cover a lot of spectrum, so almost all antennas are inefficient.
A discone is a broadband antenna & perfect for what we want. HOWEVER, all modern commercial discones that I know of, are simply a set of legs with something like a mobile whip bolted on top of them. So NOT a proper discone or genuinely broadband.
A real discone would in reality make a great mobile antenna, as the discone was designed as a broadband antenna for aircraft.
However as we cannot buy a proper commercially made discone, then that leaves two options.
One, do the research & make one.
Two. Buy a compromise antenna.
And as all commercially made antennas are a compromise, I chose to spend a small amount on a small micro mag, which for £20 or less, gives me a good plug & play antenna. Why spend more on something that does not offer more?