Maybe a different approach...
Saw this on a friends D3, unfortunately my codan is too big to be done this way, but my vhf/uhf will be done with it...
Have a look at the bar under either front seat, for seat forward/back adjustment.
Consider a wooden/metal bracket on top with cutouts for the bar.
the top brace (which runs across the 2 top bars) is held in place
by short baces at each end.
You then have the distance between the 2 bards of the seat brack to mount your radio....
its off the floor, away from the under seat vents and if mounted properly, the radio is easily removed when necessary....

