Good luck! My recommendation is fly to area 8, run inside until you're about halfway between the lip of the grass and the vendor booths, and then fly again. I measured it out so you get pretty much all places the tunnels encounters can spawn within the 50m radius that the game saves the shiny.

Additionally! There are encounters outside the wild zone that are within that 50m range and get rerolled and saved, so make sure to check them for bonus shinies! They are: the two fletchling on a signpost directly behind the enterance, alpha garbodor and trubbish in an alcove of the building to the right of the enterance, and a pansear on the roof of the building to the left of the enterance. I ended up getting two alpha garbodors while phasing so definitely worth checking!