Jon,
Horseboxes make very attractive models; one of the most attractive features is the often multi- layered effect of part planking, part panelling, framing, and the odd louvre thrown in as well.
The thumbnail shows a 7mm horsebox I made a while back, but unfortunately took no photos during construction.
Basically I cut a one piece blank out for the complete side including oversized window cut out.
I then, if memory serves, used a nominal .040 thickness for building up the side proper, chosen to give enough depth to the louvres (7mm scale).
So framing (top, bottom, end verticals and between horse flap and compartment etc) was built up with .040 thick strips. Horse flaps were cut from scribed .040 sheet and fitted into place leaving a slight gap all round. Ditto louvres. The panelled grooms compartment had the planking recessed slightly when compared with the horse flaps, so the section was first filled with .030 sheet then .010 framing added.
Strapping etc then added as normal.
I suspect the blank on which it was all built up was .060.
That did mean the total thickness was something like .100, all of which had to be allowed for when cutting the ends, and indeed disguised. Either disguised as part of a corner pillar, or covered by end detail, or rebated somehow.
Sorry that's a bit vague. But I do recall starting with the louvre panels on the principle that if they didn't turn out well, then there was little point going on. Building them up with separate louvres might work in G3 perhaps?
Jamie