Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5316422.
This platform mounts directly onto any pipe or tubular structure (standalone or part of e.g. fencing posts, gate, etc). The slats created have either a box or right angled corner, or a sloping upper surface (see pics). Any object or device attached to a plate can be positioned into the slats (plate limits are 33 cm wide, 2 cm height, depth limits depends on amount of overhang tolerated). At its current scale, the ID of the post is 5 cm.
I used it for a birdfeeder mount to a PVC pipe (an inverted/convex pan positioned on the pipe acts as a squirrel deterrent since they can't get around it) to keep the squirrels from jumping onto the birdfeeder when it was on a tree or the window sill.
I've included both right angled and sloping slat profiles in the CAD files included (Fusion 360) in case you'd like to modify the dimensions. Please let me know if you prefer it in TinkerCAD.