I'm not too fond of compression sacks. About the only use I have ever had for them is to compress bulky items such as sleeping bags and insulated jackets. When you compress insulation aggressively it can quickly reduce the loft and thus the warmth. Before I would use a compression sack, I would get a larger pack,
Loc: Portland, OR
I should think that if your skills are good enough to make a regular stuff sack that's either round or square bottomed, then it would not be too hard to add the compression components to that design. Unless you want it to be waterproof, too.
Loc: Central Texas
There are a million ways to make copression sacks, most of them are easy. Just copy something you like. On the other hand, you might want to think twice about using a compression sack. Compressing insulation tends to compromise it and it will certainly take longer to fluff back up. Lots of wilderlings use the sleeping bag/quilt and other bulky items to fill out the empty spaces in the pack and don't even use stuff sacks, let alone compression sacks which can leave a lot of empty voids in the pack bag resulting in load shifting. Besides, a pack made with waterproof material protected by a rain cover is about all the protection a down bag needs. If you plan to take it swimming, line the entire pack with a trash bag. Also, compression sacks are heavier than regular stuff sacks and always have a fiddle factor.
One more vote for going without compression sacks. But if you're dead set, this is how I "made" one:
I just used a silnylon bag that I already had. I stuffed my clothes in it, smashed it down, and then wrapped it with a few feet of light cord that I was carrying anyway, or tie out lines from my tent if I was carrying one. Remember....one key to a lighter pack is is the dual use concept.
_________________________ If you only travel on sunny days you will never reach your destination.*
* May not apply at certain latitudes in Canada and elsewhere.