Rixen-Kaul KLICKfix Caddy Post Plus Bar Clamp:

Rixen-Kaul:

The Caddy is a new version of the KLICKfix handlebar adaptor. Designed specifically for mounting onto vertical tubes such as stems or seatposts. Suitable for foldable bikes, scooters, wheel chairs, and gold caddies. It is ideal for the attachment of bigger bags, baskets or
backpacks onto vertical seatposts on foldable bikes
leaving the handlebar free for mounting lights.

Fits all tube diameters between 25-36mm.