Hmm, I just bought a TT 920T on saturday and I've got the same problem as you. I can just about wedge the TT 920T into the dash storage box without the TMC cable (sticks about 2cm out of the bottom

). However, it falls out under quick acceleration and corners.
What I've done for now is to put two velcro strips on the back of the TT (vertical either side of the fixing for the windscreen mount), and two vertical strips on the dash left of the radio. The charger cable comes out the bottom and fits nicely due to the start of the recess for the glovebox. The TNC cable is also not a problem. The balck velro strips don't stand out too much.
However...(!) The velcro strips on the dash don't stick very well (the dash material is quite slippy) As a consequence after about 30 minutes I find it came loose. I have left it stuck on for a day or so to see if it improves. Also it's a big too far left and low for my liking.
I think an alternative would be to cut a hole in the dash storage compartment to give it more height. The rubber bottom to the compartment comes off so I reckon you could cut a hole and when you don't want to use TT, you can simply put the rubber mat/bottom back over and noone would ever notice!
Only problem is taking out the compartment without damaging it. I undid the screws at the back edge but couldn't work out how to do the bit at front edge. Car is only a week old so a bit reluctant to take any risks :shock: