Many carriages for Linear Sliders on TV already. However, I could not find one for 9mm belts.
This has 55mm ball spacing, intended for a Duet Smart Effector. It is OpenSCAD, so that can be changed fairly easily. Also, the spacing of the carriage holes can easily be changed for other linear sliders.
It also features a belt tightener.
By the way, 9mm belts are almost twice as "non-stretchy" as 6mm belts. You do have to change pulleys, idlers, etc. This can be very easy, or very hard, depending on your printer.
Even though these are not exposed to much in the way of temperature, I'd recommend PETG for its strength/elastic properties.