Probably the simplest similar adapter would have been a piece of insulation from a chunk of number fourteen copper house-wiring cable. From each foot of wire you could produce about 36 of the 3/8 inch long adapters, and at about 90 cents a foot for the two conductor cable it would have been quite cheap. But those spacers are a more elegant way to do it.
I thought of making a powered propeller hat a few years ago. I wanted the propeller to turn slowly enough to be able to see the blades (2 or 3 revolutions per second or so). I programmed a PIC to PWM the motor, but I found that there were a bunch of things that affected the speed (temperature, humidity, battery voltage...) and since I was trying to run at the hairy edge, it just didn't work very well.
This must be a magic adapter. The ID and OD are the same.
A long time ago I designed a fabricated part. The machinist insisted there was a dimension problem, I of course insisted it was correct. He later came in with a handful of metal shavings.
"Here's your part" I learned my lesson.