VAR the French company made a heavy duty tool .. it was partially like a C clamp ,
so It didn't slip..
perhaps you can grind a heavy duty screwdriver blade
to resemble the Tool of sheet-metal that has proven not to be up to the job.
one with a square shank and you can clamp it in the bench vise
and have helper hold the crank, (off the bike obviously), over the tool,
while you press the whole thing down and turn the allen wrench ..

soaking the bolt in liquid wrench for a while of course..