I noticed in the photo of the USMC tartan tank, that although the stripes on the rear pleats are hanging vertically, those at the side are at an angle, with the hem forward of the waist.

It is definitely a fitting problem - the apron being flat and the wearer being rounded being the root of it, I think.

Some clever tailoring is the solution, I think - and possibly a narrower apron and more pleats, to add flare - or maybe even a shaped waistline to get the stripes to stay vertical - I am sure that many hours could be spent trying out different options trying to achieve that perfect fit.

I find that once I do up a belt, my waist is slightly compressed - so for me Velcro could be counter to a proper lie of the aprons, as it would hold them too wide - maybe if I slipped something between the two sides to stop them engaging, fastened the belt, smoothed out the aprons so they were lying flat and only then removed the separator.