Out of curiosity, would a 5x7 enlarger fulfill your needs? I don't know if I ever have seen reference to a 5x8 enlarger, although I expect there are whole plate enlargers out there.
I have a Zone VI v2 here with a few missing parts, the lamp housing is gone. It'll enlarge 6x8 inch with the right lamp housing and negative carrier. A 5x8 is fine but prefer an 8x10 or 10x10 if I can find one near me.