Any semi-stiff plane would do, the thinner the better so as not accidentally injure the little critter's legs and what-not. These days keep an unpeeled decal that has just the right thinness/stiffness properties.
The problem is that if this plane is semi-stiff, it's hard to put your hand under it without making an opening between it and the glass. And that is not a risk I'm willing to take.
So the trick is that I slide something thin (the cover itself), but then can, as a second stage, put the rest of the book there, by closing it. But true, no reason those two layers must be a part of the same thing.
Today someone posted Show HN for a catalog of problems looking for better solutions. Maybe I should submit there?
TBF, for most of the spiders that we find in our homes, the risk is only psychological, not physical. That said, I prefer not to make physical contact. My nephew though, he plays with them like they are puppies.
We keep these under every sink because they tend to appear in tubs for us. Only problem is spiders that run fast, it’s hard to capture them without injuring them.