Do you have disc or cantilever/V brakes? Also is the bike a hardtail or full suspension? If the latter then take a look at the Abus lock and canti/V brake mount adapter that Amazon has or the lock and adapter from Cantitoe Road linked to above.

Easiest mounting generally is on steel non rear suspension frames with narrow to medium tires. Ease of mounting is dependent on frame details such as seat stay diameter and the spacing between the seat stays. To fit a Swobo Dixon with square aluminum seat stays I had to make a custom mounting plate to fit the Axa lock.