Do you want to fish or just cut bait?

Fix the sprinklers or have them fixed. I imagine there cannot be more than two spraying your house. My husband will be eighty his next birthday and replaced all of the sprinkler heads in our yard.

Let it continue to spray and stain your house or use common sense. If you need a "mother may I" look up the person's phone number or ask a neighbor.

The sprinkler guard you are suggesting is small (12” x 6” x 5”) and shouldn’t be too obvious. I’m guessing that since your neighbors are snowbirds, they are not here to adjust or replace the head(s) and you don’t have access to their controller so you could do it yourself (if you had their permission). Seems like the solution you suggest could be a good short term solution until they return and you can discuss it directly with them. You do have the right to protect your house.

amen

There should be 180 degree heads in that ally , than the neighbors home would not get except if the winds blows.
Placing something on your neighbors property without their permission is not a good idea.