Locking gas cap. Stant 10501 - $10 (Santa Barbara)

Locking gas cap.
https://www.
carparts.
com/gas-cap/stant/st10501 We used this on our 2003 Ford F150.
STANT FUEL TANK CAP, LOCKING, NON-VENTED, BLACK -- 2-3/4 x 2-3/8 x 1-49/64 in.
; Includes quality lock cylinder with 2 keys; Made of sturdy and long life material; Features locking mechanism that deters fuel theft and vandalism; Easily opens with 1/4 turn of the key; Provides fuel economy and reduces fuel emission; Protects vehicle occupants from gasoline vapors.
$10