So you've cut away the pieces intended for use with old style (dual axle) radios.
Slide in the empeg bay and press out the tabs closest to the opening but that still go down behind the panel, so that the panel is caught between the lips at the front of the empeg cradle and the bent out tabs.

A backstrap tends to help stabilize the rear of the cradle from moving up/down, but shouldn't really be needed to prevent the cradle from sliding out if the adaptor is solidly mounted in the dash. In my Polo there's a hole at the rear of the DIN slots into which a rubber plug mounted on a stud screwed into the hole at the rear of the cradle fits - only stabilizing things up/down & left/right but not in/out.

The threaded hole at the rear of the cradle is M4 thread if you want to go with a backstrap.

/Michael