This is our cat run, for two cats 'boarding' here, who come from apartment living. A tunnel connects the cage to the house via a window.
It is made from aluminium tubing, bird wire, 3D printed parts, and cable ties.
I designed all the plastic parts except for the door hinges, derived from a thingiverse design.
The 3D parts are all created in OpenSCAD.
I'd intended to use plastic waterpipe for the structure, but aluminium tube (from Paramount Browns) was cheaper and lighter.
The plastic connectors are printed in PETG, though that is a bit brittle.
The Cat Run
Foot and tee
Angled tee
Five way joiner
Five way joiner, two not used
Five way corner
Four way corner