These are genuine Crosman barrel spacers.

They fit between the bottom of your barrel and the top of your main tube.

These are specifically for a Crosman 2260, but can be cut to fit any 22xx gun. They also fit the 13xx.

As it will ship, it is 12.5" long and 3/8" wide.

The two photos of the random guns show where the spacer is MISSING.

That's where the spacer goes! They may or may not actually fit those guns, they're just posted because they're showing the gap.

For your non-22xx project gun, here are some caliper-measured dimensions:
A 22xx main tube diameter is .875".
A 22xx barrel diameter is .435".
These spacers are .400" wide at their widest point.
These spacer are .139" tall at the outer edges.
The spacer's thinnest point is .020", in the middle.

The spacer can be cut with a pair of scissors, hack saw, etc and cleaned up with sandpaper or a file.