I've been having a tootle round the car park on your behalf, and I see some late model Range Rovers, before the new shape, had the radio slot moved up and level between the two air vents( presumably addressing the problems of angle and heat).

Just a thought that if you have a good non dealership Land Rover centre (We have a great one here in Leeds), they might be able to get you the parts to adapt your own fascia. I admit my version was a bit unsightly, made of ply and covered in vinyl).
Another thought struck me, my last Range Rover before the new shape was a Vogue and had separate amplifiers, I'm not sure whether the Classic had the same kit, but all my previous RR's used the amp in the head unit.

The circuit I used to run the two head units was very simple, if you could use it, send me a personal message with a fax number and I'll draw it, I'm not up with all this technology!