I use the long tether leads when working distance stays like that in public areas. Have you already introduced all the distractions you can by yourself?
Before I get to people and dogs, I'll practice stays while throwing balls and food past my dogs, running around them, jumping, hollering, playing loud recordings of dogs from my phone. I actually had a couple practice that I brought a couple pots with me and banged them together at random intervals. I'll also use the toy on the end of the flirt pole and whiz it by my dog's paws a few times while he's holding the stay.
By the end of all that, I still put a long lead on for safety, but I rarely have trouble bringing in dogs or people because he's been proofed against much more distracting circumstances.