Hi bcsteeve,
I linked the post just as just one example of many others (including many who have worked on their own layering system and gone in the same direction as you are) who have discovered that building their own mattress was a little more complex and involved than “putting a bunch of foam together in a sack” and were quite surprised at some of the complexities involved in something seemingly so simple.
In many cases they have ended up learning a great deal but what they learned also came at a cost if there are any exchanges involved.
The cover you use can make a significant difference in the feel and performance of the mattress so I would also make sure that you treat the type of cover as being just as important as the latex inside as well.
My links were meant to make sure your expectations were realistic based on the experiences of many who have gone in the same direction you have … some more successfully than others.
The “best” approach is to start with a design that you believe would work well for you based on conversations with retailers and manufacturers and your own personal experience testing mattresses. Each manufacturer will give you their “best” suggestions based on the specifics of their mattress and their knowledge and experience with their customer base but since there is no “formula” that can predict what any specific person may do best with you will also find that different manufacturers will give you suggestions that seem to be quite different (see post #8 here).
It’s realistic to “expect” that you will need to make some changes after your first combination (unless you are closer to the “I can sleep on anything” end of the scale than the “princess and the pea” end of the scale) so the return or exchange policies of the places you buy your components would be a factor to consider.
The biggest benefit of building your own vs working with a manufacturer or retailer that offers various options for exchanging layers is the learning curve you will go through and if this is part of the appeal then it can be a great project. Those who only do it to save money are often disappointed that it didn’t quite work out the way they planned once they need to start making changes from their original choices.
If it’s not about the savings then you are taking the most realistic approach. Some covers are “worth” $600 and some aren’t but a big part of the “value” / cost of any mattress is about how well you sleep on it.
A “hunk of latex” or two and a cover may be a little more complex than you realize 
Phoenix