Added v2, with rounded edges to avoid the rubber bands from being slowly cut into
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A simple, yet functional mod to turn your Thrustmaster TH8S into a sequential shifter

These two simple inserts (printed in roughly 45 minutes on a P1S) - along with some strong rubber bands - will turn your TH8S shifter into a sequential shifter, while being easy to install and remove again: Simply setup your rubber bands as you can see it being done in the photos above, and you're good to go! Just make sure to use thick rubber bands, to ensure that the shifter returns to the middle on up- or downshift.

No need to use glue or anything, the pull of the rubber bands keeps the insert in place.

Editing the model

I've attached the freeCAD file alongside the model, feel free to edit the model as you want. Make sure to add it as a remix if you do!

The file also contains another attempt of fixating the inserts by connecting them - it moves around when shifting, as the inserts aren't fixated by the pull of the rubber bands.