Quick set of unrefined models for a window stay.

The need: to allow casement windows to open to an adjustable maximum gap to stop cats falling out of windows.
 

The solution:
A hinged frame mounting point attached to the window by 3m tape or screws
An arm slider catch with tension bolt to hold the arm in place
An arm (2 lengths) to allow the width of opening required
A hinged handle catch to loop over the open window handle and attach to the arm, restricting the window opening.
2 pegs to secure the hinges

Method:
Print the parts, nothing needs supports, plug in the pegs to your chosen arm length.
Assemble the window stay
Open the window HANDLE to is open position
Slot the assembled window stay over the handle in its open position
Mark the offset location the frame mounting part needs to be placed at
Remove the assembly from the handle, disconnect the arm from frame mount for easy attachment
Mount the frame mount to the frame by your chosen method
Open the window wide and slot on the window handle catch with the ribbed side on the same side as the adjustment bolt
Thread the arm through the frame assemble
Tighten the bolt down on the desired opening width
Push the window to test the grip

Known issues:
The window handle will conflict with the frame mountif the arm is still in position when the window is attempted to be closed.
There is some play in the handle catch, use the appropriate handle catch for your window type