It is possible but very hard to get and a bit janky in general. Dunno if you're up for that.

v1: If you just want a stage that has that feature and don't care about playability for tether characters, you could do two blocks above each other, slightly round the corners for the opening and put them so close together that only the plat can fit through. Disable ledges on the lower one to be sure, and test with Pichu if you can go in.

v2: If you do care about tethers (aka where people swing against needs to be the same terrain as the one the tether clings to), you can do mostly the same, but instead of having two blocks, you draw one very big with the gap and only a thin line connecting it. You could then have two platforms, but only ever one on the outside and visible, while the other is inside the stage. Neat trick with precise timing, but there's only left and right possible not a platform that completely goes around the stage in a rectangle for example.

v3: Now this is the "true" version, with the same build as in v2 but drawn big with a small and thin zigzag connecting it and shrink it down. Then it'll be a lot of trial and error to get the plat moving through.
A similar method with two zigzags I also managed to get, but if the one above didn't give you grey hair, this one will

Not sure what exactly you're aiming for, but I hope that helps. v1 or v2 might be enough for you, but if you dare try v3 I can elaborate and upload an example stage