1. Draw a diagonal track
2. Drag a lava bubble/podoboo from the top onto the track (don't place it as it'll mess everything up).
3. Shake the track.
4. Drag a mushroom onto the podoboo/lava bubble.
5. Hit the undo dog twice.
6. Place a question mark block.
7. Add an item in the question mark block.
8. Drag the question mark block onto the lava bubble/podoboo.
9. Hit play.
If a lava bubble shows ontop of a question mark block, it means you're good to go.
10. Hit edit.
11. Drag the track off screen.
12. Make a new track directly under it so that way the question mark box will not fall offscreen and instead will be caught.
13. Hold the L/R button and highlight the question mark box.
14. Hold the ZL/ZR button and drag multiple question mark boxes on top of the real one.
At this point, you may add the other items that you want inside the question mark box.
15. Once you are done with that, drag all the boxes onto the one right below.
16. Enjoy the stacked question mark boxes.
Credits
Psycrow/JoCr2: The one who found this.
Blue Television Games: The one who made a video tutorial.