Sibylla at the Van Pelt Rare Books & Manuscript Library is saving my life.
She recommended a place in NY to buy papers with which to create the custom folios for the "guts" of the project. It is called New York Central Art Supply, and has paper goods from around the world sorted by country, which is fabulous, I think.
She recommended a conservation paper from Japan called...
OSOKAWA OHBAN - handmade from 100% kozo in natural with fine visible fibers and flecks. Smooth surface; remarkably strong; 4 deckles. Neutral pH. - 22½" x 35¼" 40 gsm $14.00
Kozo, I learned, is "a variety of mulberry plants with exceptionally tough, strong fibers, which grow wild in Japan and the Far East. Its long threads do not shrink or expand. The climate where the kozo is grown and harvested has a great effect on the qualities that the resulting sheets will possess. More than half of Asia's paper is made from kozo."
I have also ordered some more paper samples from Mohawk Paper: 160 lb. cover stock in Candlelight and Mahogany. This is from the Beckett Concept series.
The plan for the folios is to make them in a contrasting color to white/natural on a SUPER heavy cardstock (heaver than the 130 lb. Mohawk used previously), and hinge the two side flaps using conservation paper in a natural shade. This will allow me to letterpress the title onto the frontmost flap without having to change the proportions of the book or remove the tympan on the Vandercook.