The best Compression Sack for any sleeping bag should be about the same size as the stuff sack that comes with the sleeping bag.
The stuff size sack that comes with the Mountain Hardware Wraith -20 sleeping bag is: 10 x 19 inches. Here are two compression sacks that will work for the Wraith down sleeping bag:
- Sea to Summit eVent Compression Dry Sack (large) - Large: 9" x 20"
- Sea to Summit Ultra-Sil Compression Sack (large) - LG: 9" x 20", 3.3 oz, 20 L to 6.5 L
The Sea to Summit eVent Compression Dry Sack (size) Large, is the best compression sack (if you can deal with a few extra ounces) that will fit the Mountain Hardware Wraith -20 (Long) sleeping bag well. The eVent compression sack is expedition tough and waterproof, but on the heavy side. Note: the eVent medium size sack is a bit too small for the Wraith.
The Ultra-Sil compression sack uses a lightweight material, so it's not expedition tough or waterproof. When you compress the sack, you can see the threads about to break. Essentially, you need to be very careful compressing the Wraith sleeping bag with the Ultra-Sil or or else you'll rip the compression sack. But if your main concern is having a lightweight compression sack, then the Ultra-Sil will do the job.