I would re-iterate that according to military author and WY Carman, who was curator of the National Army Museum, the plaid and kilt were two separate items (not a fly plaid) in the 1790's/early 1800's and the kilt alone was worn for normal duties, without the plaid. For formal occasions, the triangular plaid was worn too and fastened at shoulder and round the waist to imitate overall a belted plaid. So a box-pleated kilt is perfectly feasible. It also makes perfect sense considering the upsurge of Highland regiments (many of which later disbanded) in the French revolutionary /Directory/Consulate/early empire war and running dry of genuine Highlanders to fill the ranks from day one, necessitating recruiting a majority of non-Highland personnel for most regiments. The first stage was this hybrid plaid/kilt/false belted plaid dress, then in 1809 the loss of Highland dress altogether for at least 5 Highland regiments (72nd, 73rd, 74th, 75th and 91st), 6 if you included the 71st (converted to H.L.I.). As it was officially put, Highland dress was “found objectionable to the inhabitants of South Britain”.