Just surveyed a homemade collimator for some of our researchers. It consisted of hundreds of extremely thin, extremely closely-spaced kapton/boron sheets.
It is elegant in its sheet simplicity. Neutrons flying straight and true pass through unimpeded. Neutrons deviating even slightly from their approved flight plan get a snootful of boron.
I mentioned it was homemade, right? Staring at this marvel, I asked the scientist how long it took to assemble. He barked out a little chuckle. “Oh! A long time. A very long time.”
Science geeks are awesome. They’re the right kind of crazy.
World's Only Rational Man 
One sheet kapton, one sheet of mylar. 2 thin spacers at each end. Repeat as needed, then clamp at edges and tension the lot. Carefully remove mylar sheets one at a time.
You need patience and a steady hand. serf labour ( undergrads looking for a good mark are cheap, only costs you a little of your beer money) is good for this.
So the off-axis attenuation comes from what? Just the cosine factor on the sheet’s thickness?
I’m guessing…yes? I know the physics of it, but not the actual shielding calculations they used. Boron has an immense cross-section for neutron capture. It takes little of it to mop up huge numbers of free neutrons. Since those sheets are about 12″x12″, it doesn’t matter than they’re < 1mm thick. Essentially all neutrons that aren't on the right path are absorbed.