The most essential part to improve the Airfix Ferret kit has been solved - the rear grille. This is SMM’s solution with a simple 3D-printed resin part. Just cut the rear deck on the join line behind the hatches, and replace.
There was the possibility of making this in etch to make it thinner, but it would have been very fiddly and many customers do not like PE.
Once the grille is in place, if desired, the resin can be made thinner with careful scraping.
DO NOT butt the grille to the rear hull like I’ve seen on built models. There IS a third gap there across the whole width. Look at real reference. The Airfix instructions are not the clearest on this.
Unfortunately, the kit’s hull walls are quite thick, but there is no practicable solution to solve this.