Rasher
If it was me I would just order the side boxes you want and the fittings - https://holan.pl/en/accessories-for-panniers/3450-pannier-mounting-pro-kit-pannier-18-mm.html separately.
You can then use your secondhand smaller boxes as the template for drilling the holes in the new boxes - so no guess work and job done
If it was me I would just order the side boxes you want and the fittings - https://holan.pl/en/accessories-for-panniers/3450-pannier-mounting-pro-kit-pannier-18-mm.html separately.
You can then use your secondhand smaller boxes as the template for drilling the holes in the new boxes - so no guess work and job done