Hi adam-in-pgh,
First of all … congratulations on your new mattress 
Don’t forget to make sure you get your free latex pillow (offered to members of this forum).
I tried all their numbers and they are all out of service so it appears that they are shut down. That would be sad because the numbers of local independents is already fairly small and in spite of the quality and value of the majority their products … some of them are simply being “overwhelmed” by the massive advertising of lower quality and value mattress brands and mainstream outlets, by the lack of consumer awareness of how to recognize better quality in a mattress (so they will unknowingly buy mattresses with much lower quality materials that have good “stories” attached to them), and in some cases by “political influences” that favor the major manufacturers and can make the cost of doing business with smaller volumes and smaller profit margins too high.
I’m not much of a carpenter so I don’t have much advice in how to build a foundation although I have seen some blueprints from others that built their own in my online travels (unfortunately I didn’t save the links). There are some good and inexpensive foundation choices in the foundation thread here but if you want to build your own either because it may save you some money or even “just because” … I’d love to hear about what you end up with and how you built it.
A typical minimum number of slats (1x3) that would be suitable for a latex mattress and give you gaps that are less than 3" would be 14 (in a mattress that is 80" in length). A stiffer polyfoam base could probably get away with less than a latex base.
I also like some of the Ikea foundations but they are made to be part of an Ikea bed/foundation option and need something to sit on (unless you want a really low bed with them sitting directly on the floor). I don’t see any reason though that they wouldn’t fit on a different bedframe where the measurements matched or on a DIY foundation that had the side rails and a center midbeam that would support it snugly without it shifting around and could keep it secure
The advantage of a flexible slat system is that it acts as an active part of your sleeping system (like an innerspring) which can change the performance and feel of the mattress (it will have more give under the heavier and more “protruding” areas of your body). This could be an advantage or disadvantage depending on how the mattress works on a rigid foundation (which is the most common choice for an all foam mattress). It would be a more common choice for a thinner mattress that needed a wider range of flexibility or was on the firm side by itself. If the slats are also tension adjustable in certain areas (like the laxeby here) then they can be be used as “fine tuning” by making the slats softer or firmer under certain areas of the body similar to zoning the support core of the mattress.
Phoenix