My bad, it's been a while since I had to re-do any electrical work involving a panel, and the summery page I was on was outdated.

Still, I'm not seeing 240v 50a breakers on either of the big box stores.

Not an electrician, but: My understanding that the updated NEC only requires arc fault breakers for 20/15 amp outlets in bedrooms.

I can't imagine too many breaker manufactures have rolled them out for anything that isn't required by code.

Your best bet is probably a 240/60amp GFCI.

Also, as /u/topcat5 said, the proper conduit installed according to local code is the best option to prevent wires being chewed.