My house has a Mastercool evap cooler, to supplement the A/C units (I live in southwest ) and it works great, however the diffuser grill is a plastic POS that looks like it came out of a 1956 single wide Champion
The home improvement stores carry all sorts of registers, but nothing big enough (20x20) for a swamper.
Can anyone tell me where I can find a nice 2 way diffuser that's made of metal, ?
Since you are already looking, look for an aluminum one.
They exist, but not at those DIY places.(supply & demand)
Call a local contractor.
He can order one for you.