I've been out in the car park, studying a Smart, as a suggestion, presuming the whole width under the HU to be the ashtray, I'd order a spare ashtray (so you can put it back to normal, when it comes to sell it) and keep the two side sections, dremelling (Is that a word!) them off, glue two sheets of plastic, top and bottom, to link the two side pieces you're left with to Empeg sled size, cutting holes for the tongues on the sled*, preferably secure the sled at the back to bulkhead or similar. You should have seen the one in my Range Rover, it looked as though the Empeg was a factory fit.

I'll have a look at it, if you're around my home or office, to come up with a more definitive suggestion.

*and to ensure sufficient ventilation

Plan B, ask Jerry at Unicar if they've done a similar fitting, mention my name, I've dealt with him for years - if you hold up your Empeg, he'll sus that you're a friend of mine! Of course, they'll want to charge a commercial rate.