This is a Remix from Thingeverse! 

I reworked litle edges for better printing.

Original Text:

I had two design goals. First a fast print. And second in one piece without support.

For the last goal the inner plate was skipped. AFAIK this surface isn't necessary in any condition. The spacer is still stiff. Even more, now a marble can drop throw the spacer if, i.e., when placed on a transparent level. Nice.

A spacer prints in less than one hour. 50 mm/s. Well, I hoped that I could do it faster. Nope. Making the walls thinner or dropping the inner supports is not a good idea.

Print it top down. The slight overhang doesn't matter, it always come out fine for me.

All wall sizes are multiples of 0.4, height of 0.2. Cura does slice it with resolution set to 0.2 very nice.

There are two thing files in the download. One has a slight wider top cutout. Useable are both. At least for me.

On my CR-6 SE both parts fit perfectly to original Gravitrax pieces. See one of the photos.

Have fun, you can never have enough spacers ;)

Ad.: Added a 5mm version.