If you want a really stable mount it amounts to drilling holes or using existing holes. Depending on your imagination, those 'holes' don't have to be in obvious places. Mounting brackets/plates/thingys can be of lots of shapes and positions.
For not so stable mounting, the options go through the roof. Glue the thing to something, use a cup-holder type mount, sand bag it, velcro, who knows what else?
A lot of it is convenience. Where can you put the thing that means you will loose some feature of your car that you don't necessarily need/want all that much? Is there a panel that could be replaced if you decide to remove the radio? One that doesn't cost an arm-n-leg?
Lots of options. You have to decide...
- 'Doc