Mini T 2.0 12mm Hex adapter - the free version!

thingiverse

The most straightforward component to create yet notoriously difficult to grasp is the hex adapter for Mini T 2.0 wheels that enables a 12mm hex 1/10th scale wheel to be mounted. This task requires only five minutes of design work, which is actually a minimal amount of time spent on it. This part has no frills and no chamfers, making it a simple, uncomplicated design. It is recommended to print this adapter at 105% to account for material shrinkage if you're using plain old PLA, as printing it at the standard setting might result in a poor fit.