I don't have enough of a memory of corned beef to know what would taste most like it, but facon could work. Maybe the Smart Bacon vs. the Morningstar kind? Homemade seitan does take a little prep work but you get a LOT of ridiculously tasty food for not that much energy; I'd recommend trying it sometime even if you don't do it for this recipe.
Out here we can get a wider variety of seitan but it's still generally not in a big block. I did see one brand that says on its box that it comes in cubes, slices, and crumbles, but I've never actually seen the slices in a store.