I teach an introductory CAD course and wanted to come up with a fun and easy project for my students to learn how to use the Sweep function.  So I decided on a Pass-through toy since they seem popular.  

This one was designed to have pretty loose tolerances so your printer doesn't have to be 100% perfect for it to still work and be fun.  I printed this one with a  0.6 mm nozzle, but it should work for a 0.4 as well.    I'm not sure if it will work with a 0.8 as the tolerance is not quite wide enough for that.

I do recommend printing the in the orientation as they are shown. Add a brim to keep them from being bounced off.  

I printed these with the end of a roll of rainbow filament, and they came out really nice.  Took about 2 and a half hours with leveling time on my Flashforge Adventurer 5MPro.  

There are a lot of possibilities for this design.  I was inspired by some of the other ones I found here and on other sites, but I didn't like the way their “ends” were open as they looked too sharp for younger kids to play with without risk of injury.  I made these so that a 4-5 year old kid should be safe to play with them.

I have included 2 different sizes of the model.  The larger one didn't finish printing correctly because I had an adhesion issue and I'm out of the cool rainbow filament at the moment.  The part that did print worked as expected and still makes a neat toy for showing how infill looks. (I would likely print the larger model with a 0.4 mm nozzle for a smoother fit.) 

I recommend using a Gyroid Infill pattern since it adds a lot of strength and it looks cool too.

Print without supports other than a brim, outside only, not inside the pattern or it might make it difficult to clean up the passage.

Have fun!

I normally list my models as free, but the whole point of this one was to teach kids how to make their own… so make your own!  (It's not all that difficult!)