This is a binary apotizer (also known as an anti-slake device), which modifies the geometry of the vanes on a reflector telescope such that it no longer produces the quadruple spike pattern on bright stars. Cuiv the Lazy Geek demonstrated this effect on his youtube channel, but the apotizers he linked are sold in Japan and cost >$50 USD. Whereas printing four of these cost me a whopping 16 cents. 

You must adjust the parameters in the .f3d file for this to fit your telescope. That's why I haven't included a simple .stl. Here's how to do this:

1. Open the .f3d file in Fusion 360
2. Select “Modify” > “Change Parameters”
3. Input the three user parameters (PrimaryMirrorDiameter, SecondaryMirrorDiameter, and SpiderWidth)
   1. PrimaryMirrorDiameter: the diameter of your primary (larger) mirror
   2. SecondaryMirrorDiameter: the diameter of your secondary (smaller) mirror
   3. SpiderWidth: the width of the vanes that suspend your secondary mirror. This controls the width of the slot that fits around the vanes. 
4. Export as a .stl and print

I had no trouble getting these to print with a friction fit, but you may want/need some double-sided adhesive to make sure these don't fall into your tube. 

The geometry is discussed and defined here. It has been slightly modified to make it 3D printable.