I've always used a 128Def/128SpD/4HP spread on mixed walls , 252Def( or SpD) /252 in some other stat/4HP on exclusively physical or special walls.
But I'm getting bugged by too many users using 252HP mons. I did some calculations and it seems having defence investment makes your Pokemon harder to KO than HP investment. So why does virtually every user I've asked their EV spreads tell me they have HP investment in bulky mons?
HP investment allows the wall to survive general hits better. Defence / Sp. Defence investment allows it to specifically take hits from that side better. A Pokémon with 252 HP / 252 Def / 4 SpD is a physical wall (unless you're Chansey). A Pokémon with 252 HP / 252 SpD / 4 Def is a special wall. Mixed walls are usually still either physically or specially biased, but they have a bit of EVs invested to take an attack from the other side of the spectrum.
i.e. a Jirachi that is mostly Specially Defensive, but with just enough Defence EVs to survive Landorus-Therian's Earthquake