Hi Alpharalpha,
Post #2 here and the links and posts in that thread have some of my thoughts about Ultimate Sleep.
I’m not sure why you’re differentiating a zippered encasement versus a sewn encasement, unless you prefer to be able to unzip and replace/remove materials. As long as the encasement is sturdy enough to help hold the foam layers in place and protect the materials inside form oxidation and UV, it should do a good job. The mattresses.net is a finished mattress so can’t be unzipped or modified. The encasement from Foam Order would allow for foam to be removed or placed inside. The comfort would be mostly determined by the foam placed inside of the covering. The Foam Order cover appears to be some sort of a stretch-knit covering.
The item you listed is supposed to be waterproof, so that’s disappointing if it wasn’t for you. There’s more options listed for mattress protectors in post #89 here and this post here.
Phoenix