The ornamental design for a roundlock for a bag, as shown and described.
1. Round lock for a bag
1.1 is a front perspective view of an round lock for a bag showing my new design;
1.2 is a front elevation view thereof;
1.3 is a rear elevation view thereof;
1.4 is a left side elevation view thereof;
1.5 is a right side elevation view thereof;
1.6 is a top plan view thereof;
1.7 is a bottom plan view thereof; and
1.8 is a front perspective view thereof, wherein the round lock is shown on an exemplary bag.
A two-piece round ornamental lock for securing a flap on a bag, such as a handbag or clutch; the handbag shown in broken lines in 1.8 is provided for the purposes of illustrating environmental structure and forms no part of the claimed design.