You'll need a compression spring - the one I used is 18mm long, with a diameter of 9.5mm.
Print the three parts, and glue the centre post (shown in blue in the CAD image) into the outer shell. Try to get this as central as possible.
Install the spring onto the post and push the (red) body into the shell. It will click over the retaining clips.
From here you simply compress the body into the shell and install a new belt.
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