Well, I have nothing against Pleistocene lifestyles, or even against the Republic of Jefferson, over which Vaughn presides, though I suspect that mastodon meat he
claims to be eating is actually elk. So to make at least a positive contribution to this ... What I like to use for neg carriers is copper-clad fiberglass electronics board. It's cheap, thin, very smooth and flat, stays flat, and is easy to machine with ordinary woodshop tools. And yes, I always use glass in carriers. Always ... even if that means hiring Neanderthals to polish down crystals (which have become quite expensive, since modern day-chemically-induced Neanderthal minds have ascribed certain magical qualities to them). About the only down side of electronics board is that you need to prime and paint it black. And obviously you need to buy the undrilled type. A very distant second would be thin-veneer hobby plywood, epoxy-pickled. But since D2 carriers are abundant and dirt cheap, I simply filed mine a little bigger for full-frame coverage. The standard glass still fits.