Go to any engineering supplies company, buy lengths of 12mm high tensile studding (threaded bar). Cut to required length and use HT nuts to form the head of the bolt and HT nut at the other end.
Now you can make bolts exactly the length you need.
For extra safety, use locking nuts for the 'head' and if you're
really a 'belt and braces' man screw the locking nut a couple of turns futher up the thread and then 'peen' the end of the stud. (Peen: bash with ball (peen) hammer till end flattens slightly, stopping the nut from coming off
)