This is a custom-designed "wedge" that improves the tilted angle of the Gibraltar 9608MB 9000 series bike/tractor-style drum throne (aka musician's seat, guitarist's stool, or drum chair).

A common complaint from reviewers of this chair is that the cushion's slope tilts you too far forward, throwing you off-balance.

This wedge effectively and securely raises the front end of the chair by ~10mm for a more stable seating position.

It's a fairly simple print. I printed mine with 0.28mm layer height, 25% infill density in octet pattern (for vertical integrity), and no supports.