I found this one, and I've tried about 6 or so, this the best so far.
The strap "riding up" in the back seems to be a chronic problem, with the chinstraps too. But with the nasal pillows, if you naturally breath thru your mouth, you need a chinstrap.
Try to work with the velcro straps, to sort of cling the chinstrap to the mask strap, try letting it stick to your hair a little, the way you overlap the velcro.
Let us know how it goes, it takes time to get the comfort, but it's doable.
I encourage you to checkout the Ultimate Chinstrap - http://www.ultimatechinstrap.com/ . After failing to find a chinstrap that really worked well for me, I created this chinstrap. In regards to the strap "riding up", this is not an issue at all with this chinstrap. Also, unlike many of the other chinstraps available, this strap doesn't require you to have to "crank" it down so tight that it puts a lot of pressure on the lower jaw making it very uncomfortable. Like I said, I tried several chinstraps with little success but the Ultimate Chinstrap truly does work. Let me know if you have any questions.