When I saw the subject I was going to suggest a place to buy giant pieces. For a reasonable piece, yes, fabric stores are definitely the way to go. The idea of soldering in a divider sounds good, too. If I were building a Cmoy, I might go with the foam. I like the idea of the foam keeping things stable.
Home Depot sells this stuff called "Frost King - Vinyl Foam Weatherseal Self Stick Tape" that works great (Lowe's and other hardware stores probably have it or something equivalent too). It's available in several widths - I got the 3/8" wide - 3/16" thick; it's sticky on one side. Perfect for fitting batteries etc in mint tins. A few bucks will get you a lifetime supply.