This bookmark template can be used to create other bookmarks. It is based on Schrittmotor's Peeking Cat Bookmark. 

1. Modify the bottom portion as desired, or print this out directly
2. Subtract the “Bookmark Template Top negative” portion from your desired bookmark top. This can be done using OpenSCAD's difference() command or using another program's subtraction command.
3. Preview, Render, and Export to STL

Using a 3DBenchy.stl as an example, the command in OpenSCAD would be:

difference(){
import("3DBenchy.stl", convexity=10);
import("Bookmark Template Top negative.stl", convexity=3);
}

Note: You may need to modify the imported str design a bit to get the size just right. This can be done with the following commands:

translate([x,y,z]
rotate([x,y,z])
scale([x,y,z])