How it is made. The tray is a 6 mm plate with a 4 mm lip, the cup recess 88 mm across and 4.5 mm deep, the phone slot leaning back 12°. The hinges: seven knuckles along each long edge, alternately part of the tray and part of the flap, on a 4 mm pin with half a millimetre of clearance — printed flat, all one piece, and freed with a firm fold the first time. The flaps are 70 mm deep. The strap is a 12 mm ladder with ramped teeth every 5 mm, 200 mm long; it threads through the buckle on each flap and a pawl on the buckle clicks over the teeth one way and lifts to release. It closes on armrests from 120 mm to about 280 mm round.
Print and test. The shown preview is the print — everything flat, the strap beside it. A person checks the hinge and the pawl before it prints; if your first one is stiff, tell us and we tune the clearance for the reprint. The same buckle and ladder are our latch for anything that needs cinching round a girth — a post, a pipe, a tree.
Sizes. Standard 240 × 140 mm, $28. Wide 300 × 160 mm, $36 — two cups.