What I do (this may defeat the purpose of saving the $70 on a slinky) is buy a whole bunch of blankets in different shapes/styles. Each one fits a little differently, so as I alternate them, they never consistently rub in the same spot. This also makes me make sure that I'm frequently having them laundered, so I don't get any stinky/mildew/mod/gunk buildup.
It sounds kind of expensive, but I just buy things in the off-season and my horse takes a pretty large size, so usually the "left over" sizes will fit him.
I've also had show sheen work fairly well. Do the blankets that rub your horse have nylon lining? Felt/fleece/wool? I've heard that a silky material (such as nylon) is more likely to glide across your horse's coat and not catch/pull/rub like others.
Also, you can just buy a shoulder guard instead of the full slinky to save some money. You could probably get one on eBay for like $20 (including shipping). http://sporting-goods.search.ebay.co...40QQsacatZ3153