Hey, thanks for the responses!
I have a new horse (yay!!!! Love her), she's a QH mare, but a bit long (so the synthetic one might not work well) and I'm using my treeless on her, my saddle was made for my high withered STB, she is not high withered so I'm assuming this could be the problem....when going downhill my saddle slips forward- yikes. I hate to give up my saddle, plus I still use it on my other guy, so somebody suggested a crupper. I hate to buy another piece of equipment, in case it doesn't work out, I'll put it back on here.
I sold my saddle, had the saddle fitter out and still ended up with a saddle that slips downhill, so I guess it's my mare's conformation.
So, if anybody still has an old crupper they're not using, I'd love to have it.