Hi LisaL0115.
Welcome to our Mattress Forum! 
Because of the uncertainty involved in “replicating a mattress” the risk of getting it just right is much higher… unless of course all the components, layers and design or your DIY “build” is an exact replica and matches in every way the mattress you have tested or slept on (in effect resulting is the same mattress). “Replicating” a mattress component by component in view of a DIY mattress based only on “specs” without the specific guidance of a manufacturer/retailer that is familiar with your reference mattress and materials is unreliable because in most cases:
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- Consumers would tend to rely only on the ILD of the layers or other more basic specs which by themselves can be very misleading and inaccurate because of any design specific properties, response, or factors that can make a significant difference in how a mattress feels and performs.
- Consumers don’t have access to all the information they would need about either the mattress they are using as a reference point or all the components they are gathering to make a meaningful comparison and even if they did (which is rare) most consumers wouldn’t have the knowledge or experience to be able to “translate” how the many seemingly small differences between two mattresses may either accumulate or offset each other and “feel” in real life.
- Also sometimes what a consumer “thinks they need” is often different from “what they really need” and this is better done with the guidance of a knowledgeable and experienced online retailer or manufacturer will usually be more focused on “what they really need”. [/indent]
The “target” with these types of choices is generally how closely a consumer believes one mattress will match the one that they are building which is often very inaccurate, this is why I always suggest that those who take on a DIY mattress replication project work closely with a manufacturer who is familiar with all the components of the “blueprint” mattress .
Avena is a High Resiliency polyfoam which is high quality and durable material introduced by Carpenter in 2010 that has “latex like qualities”. It is more costly than conventional polyfoam but not as costly as many more premium foam materials such as higher quality memory foam or latex. You can read a bit more about Avena here. To find something similar in terms of quality and durability you may wish to check out the foam suppliers listed in the same thread in Post #4 here ( a few links were recently updated) with latex being the closest in feel, but you may wish to talk to each of the manufacturers you are considering purchasing the component from to see how closely they compare.
Arizona Premium Mattress (member of this site) has a cover that would fit your criteria and generally a large selection of mattress covers, bamboo, organic wool cover, organic cotton. The zipper is made with Teflon and zips around all four corners making it easy to assemble. Some models come in sizes from 6" to 15"
You may also wish to contact some of the manufacturer members below who make their own covers to see if they would be willing to adjust mattress size for you.
Latex Mattress Factory (member of this site) has a removable and washable 360-degree zipper cotton cover from 2" to 12" in thickness.
Memory Foam Comfort(member of this site…Canada) carries zippered and removable Tencel (eucalyptus), Bamboo, Teddy (terry cloth) or Wool quilted 100% organic cotton sizes 4-12".
Sleep EZ (member of this site) has Natural and Organic cotton & wool Zippered Mattress covers between 6" and 12" in thickness
It seems that you are in the latest stages of your “built” and hopefully some of the information listed above will be assistive with your DIY.
Phoenix