I used to have a TS440SAT in my passenger seat (company truck, never a passenger). I made a "base" using 3/4 plywood. I made a "box", the top was level with the world, but the bottom was angled with the seat. In the truck I was using, this was about 3 inch difference between the back of the seat and the front of the seat.
I then drilled four holes in the top surface for the radio feet to slip into, this kept the radio for sliding. I also allowed room under the HF radio so I could put a V/UHF rig and external speakers in the mount.
I put eye hooks into the box top on either side of the TS440S, I held the radio down with "bungie cord".
I ran the seat belt thru the box so that if I did need to make a quick stop, every thing stayed in place.
This worked out nice for me for several years.