Garmin Edge 540 low profile stem cap mount
A stem cap mount replaces the top cap on a 1 1/8 inch steerer and puts a Garmin Edge 540 or 840 directly above the stem, the tidiest position for a commuter, gravel or Brompton-style bike where an out-front arm would get knocked. The 32 mm disc is 6 mm thick with a countersunk hole for the standard M6 top cap bolt, and the quarter-turn socket on top matches Garmin's four-tab layout with 0.2 mm clearance so the unit clicks in firmly. Print it socket-side up in PETG at 0.2 mm layers; the flat underside needs no supports.
Because the socket height and disc diameter come from your stem rather than a random model, the computer sits level and does not foul the bar. Type your steerer size and desired tilt into SpeakCAD, preview the cap free in the browser, tweak the socket angle in the chat, then download the STL.
Create a bicycle stem top cap 32 mm in diameter and 6 mm thick with a countersunk 6.5 mm centre hole for an M6 bolt and a Garmin quarter-turn socket on top with four tabs, 12 mm tall, 0.2 mm tab clearance and 2.5 mm walls.