Advice on a few levels. Sorry if I go over things you already know, but someone may find value.
1. Pre-roll before you pitch: Lay down and feel where the ground will roll you prior to pitching your shelter.
2. Pack your edges: the tops of your Boots, tucked under your mat at knee or thigh level, make a great anti-slip barrier. Tomorrow's socks and a pair of gloves rolled and tucked just under the edges at chest height help too, and are warm and easily found.
3. New gear! If you'd like the excuse to upgrade, I have had great comfort from expeds. The downmat 7 has integrated pump and down filler, but you pay a weight and bulk penalty. The Synmat 7 is much lighter and plenty warm enough for sleeping on snow. You do need a pump or drybag with a valve called a 'schnozzle sack" to inflate it, but it serves to keep your gear dry as well.
Both have higher edge baffles to combat rolling off. Downside to them is the baffles run full length, so if you're sloping downhill you may slide down the mat a bit.
Sorry for the long comment, but I enjoy a good sleep systems and got excited.