Well, looking at the pocket that used to sit in the lower
DIN slot in my VW Polo '01, there's what looks like a part
number, 6XO 857 058 (conforms to Hella part numbers too...)

I think it's marginal if this part will fit in the sled
(judging by measurements on the piece and on the empeg.

Depthwise - no problem, the part that extends back to grip
the hole where the screw/rubber plug on a normal HU goes is
easily hacked off and the pocket itself is ~10mm shallower
than the empeg (MkI) metal chassis.

Width - no problem after possibly trimming some small plastic tags

The problem is height - this one is very marginal,
measurements makes it ~1mm higher than the empeg including
the lid screws even after trimming off extending tabs.

I'll chop it up and get back with the results...