Are there clips or "cotter pins" which can't get knocked out by accident?
Our lawnmower shipped with pins that look like this. 



I took them off because I thought they were used because they were cheap (somebody at Sears was laughing all the way to the bank over the 1" less of spring steel rod they use per fastener.  I switched to the proper kind, the ones you show in OP.  

They fell off at such a rate that I soon ran out of them, and was forced to go back to the original (here above) kind.  They have been reliable.  

Hard to take off, but that appears to be the necessary compromise.