Harness and collars, raw spots

Having trouble finding correct gear for this beast of mine.
She's about 35# and built skinny. Has the body style of a German Shepherd but petite and short, thin hair.
Her head is just as skinny as her neck, so even a too-snug collar can be pulled off easily.
We have tried about six hundred harnesses, it feels like, and own five. Only one of ours has a "proper fit" where it lays comfortably secure on the body. I prefer not to use it because it says, in big letters, Service Dog (and she is not officially trained as one yet.) Most harnesses, if going by size charts, don't fit secure (big belly band gap, or too tight on neck, etc.) and going up or down a size makes an impossible fit. She can slip out of every harness we own with a simple body flick, to give an idea of the severity.

Worse yet, we MUST acquire a good fit harness because everything we own is rubbing her raw (armpits and neck) and if my children open the front door she likes to take advantage and go chase squirrels. We are big on hiking and are often with her off leash in the yard, woods, and at family's houses, it is paramount she has tags on her every waking moment.

How do I find a better fit?! I've gone to every pet store within a 30 minute radius without luck. Size charts have proven useless.