you'll unfortuantely find that car covers are very un-securable based upon the nature of fabric & securing method/ hardware

a person whom wishes to 'explore' underneath the cover may easily pull the cover over the front or back of the car and/or cut around the fabric where any securing hardware is attached to gain access

Formerly, I used an MB car cover with a cheapie lock and cable in an overnight underground parking facility with my '94 E320 Coupe. The underground facility was attacked twice (break-in;s to ~12 cars ea time)- - the Coupe was left unscathed (cover intact).

For obvious reasons, it helps reduce the chance of break-ins to keep an auto interior locked & 100% clutter free- no sunglass, no papers, no CDs, no tapes, no clothing, no phone- - nothing extraneous visible (those items may be secured in the trunk).

I recall that there are state-of-the-art super sensitive vehicle alarms that may provide some extra security for you, however- - the general rule still applies- - if a thief wants to get into your car, they will.

good luck
-fad