• Maybe make a version that goes around seat posts? After all, glue and ball bearings won't do much to stop a determined thief with a hack saw.

    Ditto. There are DIY solutions, but AFAIK no specific product yet that protects seat and seatpost - that stays on when you ride that is.
    Instead of fighting for a corner of the lock market you could create a whole little niche for yourself with something a bit smaller and neater.
    I'd pay up to £30 for something nice, light and user friendly to secure seat+seatpost.

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