I'm in the midst of starting to make some 14x17 film holders. I thought I'd be doing this a couple of years ago, but stuff got in the way. But back in 2017-18, I did a lot of searching for materials and others' experience, and I've now gone through the same process.

For dark slides (and septums in double-sided holders) folks have used and recommended metal (steel, spring steel, aluminum; even titanium [though that may have been a joke]), plastics (styrene, acetal [Delrin], and others), and even thin plywood. The one I considered most strongly back a couple of years ago was XX Garolite. After lots of thought and research, that's where I am again.

I have made 4x5 and rollfilm holder dark slides from thin steel and they worked fine. That's my only experience. But I've worked with both steel and aluminum for plenty of other projects, and find them unacceptable. Steel is heavy and expensive. Aluminum is soft and expensive. Styrene is inexpensive but weak in larger sizes. Acetal is, again, expensive. In fact, expense is the primary reason I came back to most favoring the Garolite. But it is also lightweight, of good rigidity, workable with the tools I have, available in a variety of sizes and thicknesses, AND it sure seems to be what the dark slides in some of my old holders are made of.

However, in more than one place, I've read that it is not always opaque. Obviously, this is bad. But here's my question: would it not be a simple solution to just spray paint it?

There's no way I could attempt this project without doing my best to limit the spending involved. I just had a discussion with my wife about just how much I already spend on my hobbies, even though some of them actually bring in some funds. I stopped her by suggesting I could take an interest in bass boats, or a Harley. Still, the low price of Garolite makes it a really attractive option, and I'm not seeing an issue with guaranteeing opacity through a rattle-can of black paint. In fact, if I choose to do this, I can buy the less expensive brown stuff. Though maybe I should, anyway? (I guess that's another question.)