Measured with a "sewing and knitting gauge" (sort of one sided calipers), they are 11mm wide, but are flatter on the top, maybe 9.5mm top to bottom.
If I fit Vaude bags with the 10 mm spacer (
https://www.vaude.com/en-GB/Products/Activity/Cycling-Tour/QMR-Reduction-parts-2.0), the fit is very snug, but needs a very firm tug on the release to get them off, so I use the 13mm spacers which is a bit sloppy, but holds it firm enough.