Need an F11 fixer for my Topeak saddle bag.

I've just tried to move my Topeak saddle bag from my old bike to a new one, but can't as I need an F11 fixer, which doesn't seem to be available now. Does anyone know where I can get an F11 fixer? Thanks.
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Or does anyone have another solution for this? Last time this happened I just upgraded the seat post to Tifosi Carbon, but this one is Cannondale 25.4mm so my options are limited and anything higher quality is likely to have another two-bolt clamp.
Thanks!
2020 Ribble CGR SL
2019 Vitus Vitesse Evo CRS Disc
probably 7pm before I can confirm.
mike