THIS Icetoolz one is good value and should do the job.
Take it you don't have any flats for a 15mm spanner?
If it's well tight use a drift to give the end of the pedal axle a few sharp taps to free off the threads a bit.
Make sure the allen socket is cleaned out so the key gets a solid fit and maybe tap it in a bit with a hammer to seat it well.
If you need more leverage slide a ring spanner or some tubing over the end of the allen key.
If you still think you may round it off - heat the end of the crank arm with a gas flame (a camping stove or mini blowtorch) and the alloy arm should expand quicker than the steel(?) pedal axle making it easier to loosen.