If you are REALLY interested in making a custom mount for the area you have shown in the picture, and you are pretty handy, here it is:
1. Buy some florist's foam (dense green can get anywhere)
2. whittle it down until it fits loosely into the area
3. Fiberglas the outside of foam on all sides except back and bottom
4. Test fit! Get it tight!
5. Sand, prime and paint fiberglas shape simigloss black.
6. Chip out foam
7. Mount face to new mount
I have done this before, especially for aftermarket GPS mounts. Works well and easy to take out leaving no marks when you want to sell or change radios or something. Takes about a day, off and on. THe brand of fiberglas is called Duraglass and is a resin with chopped up fiberglass mixed in. It works like body filler except it gets really hard when cured, which takes about an hour. You can actually grind it to shape for the rough initial work.