Hmmm, I've run try-all cranks with the symetrik and an SL for years with no problems. Is it worth putting a pic up? As long as theres no washers/spacers in there and the crank sits nicely in the cut-out I can't see where there would be an issue.
If you put the bash on the crank nicely in the cut-out, put the freewheel on and tighten it up by hand so there is nowhere for the bash to move, that should work to stop it jumping out of the cut-out?