I designed this button box to be used with OMP Trecento Steering Wheel (it might be suitable for other wheels as well). For the reference OMP Trecento has OD of 300mm and PCD is 70mm.
Adapter for Thrustmaster T300 is made by RamjetX https://www.thingiverse.com/thing:2630578
Shifters are made by AMSTUDIO https://pinshape.com/items/47862-3d-printed-diy-magnetic-shift-paddles
Components that I've used:
M3x30mm bolts and nuts