When breeding a Pokemon with a regional form, there is no way to choose what form you get, it only gives you the base form. In this specific case I was trying to breed a Galarian Yamask. I used the following combinations:
x2 Runerigus
Runerigus + Cofagrigus
Runerigus + Ditto
Runerigus + Koffing
In all instances I tested with no everstone, an everstone on the male parent, an everstone on the female parent, and an everstone on both, with no difference. The wiki mentions choosing the regional form preferred via "yellow arrows" in the Day Care UI, but I don't see them at any point while setting up the breeding process.